I kept asking them just to return funds to the same address from which they received the payment. And they keep repeating that they can do it after I send them all "supporting documents"
There's nothing much you could do since their ToS explicitly stated that. But i found out such ToS didn't exist last year,
https://web.archive.org/web/20210613120712/https://openchange.cash/t%D0%B5rms_of_service/.
Here's an archive in case some message gone,
https://web.archive.org/web/20220701120523/https://www.bestchange.com/openchange-exchanger.html.
The funds have an AML score of 94.89999999999999 according to my system. It is clearly 100% money laundering and they had all the right to take your funds if they have any kind of AML policy.
Could you explain why you said "It is clearly 100% money laundering"? While 94.89... is quite high, i wouldn't say that when doesn't meet 95% or 99% confidence interval.
Chipmixer is not recommended to use as mixer, it is detected 80-85% of cases. Try to use coinjoin wassabi, but there you have to extremly careful (there are some settings you need to apply) for a real mix.
Aside from privacy problem with today's wasabi coinjoin, i don't see how it could solve OP problem. While wasabi coinjoin might have lower percentage on your system, it's known centralized exchange also reject coin from wasabi coinjoin.