Severe car seat aversion - HELP

Please help. We are desperate.

My three year old is experiencing SEVERE car seat aversion and I feel like we have tried everything available to us. This has been going on for nine months and it's currently the worst it's ever been. She has no issues with the car or the car seat itself, but she comes completely undone when the straps are tightened.

She isn't able to sit comfortably unless the straps are extremely loose, roughly 5" inches away from her chest when extended - this is obviously not safe. Any tighter than this and she will fight: kick, scream, hit, throw things, nearly hyperventilate and/or vomit, and she will try to escape. She will rub her thighs and shoulders raw attempting to escape while thrashing and screaming as if she is quite literally being murdered. She repeated over and over that the straps are too tight. She will do this for entire car rides. Additionally, she can't wear ANY clothes in the car seat. Not even underwear.

It feels like we've tried everything:

• distractions: snacks, toys (horrible ones with lights and buttons and sounds), books, screen time, music, Yoto player, etc.
• bribery
• different strap covers for softness and in her favorite color
• different car seat (fancy rotating one and basic bare bones one)
• switching to forward facing
• body work: cranial sacral
• spoke to pediatrician: she suggested we try play therapy at home
• brought car seat in the house for a while to attempt to desensitize her that way

Nothing is helping. This week she has started slapping and pinching herself during car rides and stating that she needs to pee/poop so that we'll let her out to use the toilet. She doesn't actually have to go/hasn't gone any of the times we've pulled over to let her.

She isn't big enough for a booster seat, but I really think that could help us out here...

FYI we have a Nuna Rava and we've tried the Evenflo Revolve 360° and a Cosco convertible sear.

We are so desperate for help with this. We really can't just not drive. I am willing to try just about anything at this point.

Thanks in advance.