Advice/Suggesting for managing self-injurious behaviour during meltdowns, but also a lot of medical needs to be consider

Hoping to some advice on how to manage/reduce the self-injurious behaviours of a fantastic young person I support, but who also has a lot of complex medical needs that need to be taken into consideration.

The person I support is in there 20s, mental development assessed at 18 months, wheelchair bound, cerebral palsy, has a PEG and a Stoma, epileptic, and  is non-verbal with severe autism and learning difficulties.  They can use selective words but this is not always contextual, they have selective vocals to indicate preferences, and we also use personalised cards on an activity board, and now and next board, and a choice board as a way of communication.

The biggest issues is when things are not going their own way, they exhibit controlling communicative behaviours  e.g. if we are on a walk and stop for a minute to take a break when they wanna keep going, or  if we don't turn the radio over the second they ask, or ask them to wait as your concentrating on driving. We often get behavioural coughing and trying to make themselves sick, unfortunately this can trigger seizures. Sometimes they may also try to bite themselves or us, scratch us, or they try to pull off their stoma bag, and have on occasions gone for the PEG. Sometimes this behaviour is also noted on processing transitions as this can be a big trigger, but this is  prominently used as an opt out.

We have a selection of tactile motivators/rewards that do help minimise behaviours, the biggest motivator is the car, this person could be in the car all day everyday, just genuinely enjoys it.

However this is where I need the help/advice, on occasions even though we have prepped them, have motivators/and rewards. Getting out of the car some activities, such as swimming which in their best interest, its physiotherapy , they have done it for years and they enjoy once they are in there. We get the coughing, the intentional sickness, the clawing, biting and pulling at the PEG and Stoma.

At this point verbal prompt/instruction has no effect. And physical prompts are limited.

In an ideal world I would let them ride it out until they have calmed themselves down, however the self-injurious behaviour e.g. pulling PEG out , tearing the stoma, coughing themselves into a seizures are very dangerous and potentially life threatening, and needing surgery.

 So as a result we have to try and stop them from doing it as they do not understand the consequences of this action, but then the behaviours are redirected onto us, our arms scarred all over, with nails and bite marks, and hair pulled etc In trying to stop them from seriously damaging themselves, what choice do we have? we have arm guards, and there is always 2:1.

But I wanna work on how to help the individual deal with it better. I don't want them to hurt themselves, its been suggested by psychologists in the past the self biting is a release, which I could understand, but its the PEG and Stoma that worries me.

I fear this has also become such a learnt behaviour in some incidences , they know that's how to get their way. If it was an activity then fine we can just abort and  try something else, but when its hospital appointments these are things we cannot walk away from, they need to be done so that they stay well.

Any advice?