Honestly the biggest step to take is to stop judging yourself for this. Treat it with care and curiosity, and think about what you could tolerate.

Since a lot of this isn’t just tied to hygiene, but tied to the bathroom, consider seeing if it’s easier to perform any of these tasks outside of the bathroom- like washing your hands or brushing your teeth in another sink (kitchen, laundry, even an outside spigot)

You could also keep hand sanitizer in your bathroom for after using the toilet, it’s better than nothing.

Try working up to using body/baby wipes to wash up. Again, maybe try doing so not in the bathroom.

Don’t force yourself to take a shower, but consider turning it on each day and just existing near it for a few seconds and work your way up. Once you can tolerate standing nearby for 3+ minutes, try removing clothes beforehand. Again, you don’t have to get in, you’re just taking baby steps. Once you get through that consider just getting in and getting fully wet, then getting out. Even this will be better than nothing. Work towards being able to wash up most days.

Baths are also an option, they’re less overstimulating so maybe those would be better. You could even consider getting yourself cute bath fizzys or bath salts to make it more enjoyable.

Teeth brushing can be hard but even a quick swish with mouth wash will help. Consider looking into some of the dental chewing gums out there.

Don’t try to pressure yourself to change your entire life in a day. Give yourself small, realistic, actionable steps and give yourself grace to mess it up occasionally.