OP I can totally relate. we had similar issues when our children were younger.
My DSS is autistic, so is my DS so I have experience of parenting and step parenting autistic children.
My DSC have never been given many boundaries at home, they were allowed to stay up until very late, they weren't encouraged to clean their teeth or wash, they just ate snacks rather than meals, I'd go as far to say they were pretty neglected. They're now teenagers so they're pretty self sufficient, still not ideal but they're ok.
We had so many issues for years over the different boundaries, or the lack of them, and then the impact it had on my children. Bedtime was the main problem and just general behaviour, swearing etc.
DH often wouldn't deal with things as well as he should have because he was scared of upsetting DSC and them not wanting to come anymore (his ex was prone to stopping contact over the smallest of things).
After 13 years of step parenting my advice would be to pick your battles. Decide which boundaries are most important to you and push those, get DH on side with you, and let the other things go. Especially with autism being thrown in to the mix, it's even more important to pick your battles because if you fight them all you'll be drained.
Most kids are good at learning different rules/routines in different settings - eg most children know they have to behave a certain way at school, it's not too late to gradually put in place certain rules / boundaries. You could also try visual timetables and/or social stories to help her know what's expected.
I did wonder if the chocolate for dinner is due to issues around food? A lot of autistic children end up eating unhealthy food because it's all they will eat and parents would rather they eat chocolate/crisps/biscuits than starve.
most importantly though - Your DP needs to support any changes 100%, DSD needs to see you as a team or else it won't work.
If your DP isn't supportive then seriously consider leaving, in all honesty part of me wishes I'd left at the beginning. I kept telling myself things will get better, they did eventually but it took about 12 years, a lot of arguments between me and DH, a lot of heartache, his ex has been awful to me. Life probably would have been a lot easier if I'd walked.