Autistic parents, how do you know if you will cope with a second child?

I'm not sure I have the most supportive thing to respond here, but as some others have pointed out, maybe you're partially looking for a negative response in some ways. I suppose what I'm saying is that your worries are indeed valid. Whilst no-one else can ever make the decision for you, every family is very different and like you say, you will never know how you're going to find things until you're past the point of no return. I'd say that a key thing would be to have more in-depth dicsussions with your husband beforehand to ensure that he understands the potential risks and your concerns and to ensure that he would be in a position to support you all in whatever way you may need.
I came here just to share my experience in that I hvae 2 children (son currently aged 3 and daughter currently aged 1)
Whilst I have known myself since teenage years that I was autistic, it was only after the birth of my second child that anyone else really flagged it. For me, my diagnosis was triggered by the psychiatrist I was seeing as a new parent to my second child.
What you have described in your original post regarding your fear that a second child may be too much for you, is exactly what I have experienced, however I didn't put the consideration in beforehand.
I have now very much ended up in the situation of having 2 absolutely wonderful children, but I do find myself regretting that decision, as I feel that it has taken me to the point of really struggling on a day-to-day basis after having been able to manage my autistic traits the majority of the time until now.
As such, I have now ended up with additional mental health challenges.
My husband very much has to take the brunt of things in terms of supporting us as a family of 4, working full-time as well as most of the housework too.

The other thing that I will add, is that I am now also noticing a significant difference between my son and daughter and have just had a conversation with his nursery about potential ASD traits in him too. I don't know if I struggle more with him because of his age, or because of this. Someone else's comment made me consider the fact that you currently have a daughter. It's well-known that autism affects girls and boys in different ways, and for some reason at the moment, I'm feeling more confident for supporting my daughter growing up as a (potentially but not yet suspected) autistic girl, than I am for supporting my son growing up as a (suspected) autistic boy. Partly, I think this comes down to personal experience and knowing how my autism has impacted my life for so long, without really knowing what that's going to feel like for him growing up with autism as a male. Obviously you cannot predict or choose the gender of another child so it may be worth considering that they wouldn't necessarily be as manageable as your first child even if they were alone, in addition to the fact that you'll have an extra person to consider with everything.

The age gap question is something that lots of other Mums ask me about because of my children being quite close together. There are many pros and cons to every age gap and I don't believe that there's ever a perfect age gap. You may well find that as your daughter is already slightly older, things are more manageable than they have been for me, as both of my children are very much still dependent on me and haven't yet fully reached a stage of independence. Hence, one of my most common difficulties and overwhelm can stem simply from the need to toilet/nappy-change at a public place, or simply getting them into/out of the car. Where am I putting the other child whilst toileting/changing each of them, where do my children go when I need to go to the toilet whilst we're out? Who do I get out of the car first? Who do I put into the car first? Whichever option I go for always leaves the other child unrestrained for a short time. I still haven't worked out which option is best(or indeed if there is a best option) and change my mind depending on the situation, but it's one of the little things that crops up such a lot that causes a great sense of overthinking and overwhelm to me.

I'm not sure I have the most supportive thing to respond here, but as some others have pointed out, maybe you're partially looking for a negative response in some ways. I suppose what I'm saying is that your worries are indeed valid. Whilst no-one else can ever make the decision for you, every family is very different and like you say, you will never know how you're going to find things until you're past the point of no return. I'd say that a key thing would be to have more in-depth dicsussions with your husband beforehand to ensure that he understands the potential risks and your concerns and to ensure that he would be in a position to support you all in whatever way you may need.
I came here just to share my experience in that I hvae 2 children (son currently aged 3 and daughter currently aged 1)
Whilst I have known myself since teenage years that I was autistic, it was only after the birth of my second child that anyone else really flagged it. For me, my diagnosis was triggered by the psychiatrist I was seeing as a new parent to my second child.
What you have described in your original post regarding your fear that a second child may be too much for you, is exactly what I have experienced, however I didn't put the consideration in beforehand.
I have now very much ended up in the situation of having 2 absolutely wonderful children, but I do find myself regretting that decision, as I feel that it has taken me to the point of really struggling on a day-to-day basis after having been able to manage my autistic traits the majority of the time until now.
As such, I have now ended up with additional mental health challenges.
My husband very much has to take the brunt of things in terms of supporting us as a family of 4, working full-time as well as most of the housework too.

The other thing that I will add, is that I am now also noticing a significant difference between my son and daughter and have just had a conversation with his nursery about potential ASD traits in him too. I don't know if I struggle more with him because of his age, or because of this. Someone else's comment made me consider the fact that you currently have a daughter. It's well-known that autism affects girls and boys in different ways, and for some reason at the moment, I'm feeling more confident for supporting my daughter growing up as a (potentially but not yet suspected) autistic girl, than I am for supporting my son growing up as a (suspected) autistic boy. Partly, I think this comes down to personal experience and knowing how my autism has impacted my life for so long, without really knowing what that's going to feel like for him growing up with autism as a male. Obviously you cannot predict or choose the gender of another child so it may be worth considering that they wouldn't necessarily be as manageable as your first child even if they were alone, in addition to the fact that you'll have an extra person to consider with everything.

The age gap question is something that lots of other Mums ask me about because of my children being quite close together. There are many pros and cons to every age gap and I don't believe that there's ever a perfect age gap. You may well find that as your daughter is already slightly older, things are more manageable than they have been for me, as both of my children are very much still dependent on me and haven't yet fully reached a stage of independence. Hence, one of my most common difficulties and overwhelm can stem simply from the need to toilet/nappy-change at a public place, or simply getting them into/out of the car. Where am I putting the other child whilst toileting/changing each of them, where do my children go when I need to go to the toilet whilst we're out? Who do I get out of the car first? Who do I put into the car first? Whichever option I go for always leaves the other child unrestrained for a short time. I still haven't worked out which option is best(or indeed if there is a best option) and change my mind depending on the situation, but it's one of the little things that crops up such a lot that causes a great sense of overthinking and overwhelm to me.