Extinction struggles - baby falls asleep and immediately wakes up crying

Hello! Hoping someone will have some input here.

Our son is 7.5 months old and not a great sleeper. We’ve decided to sleep train him using full extinction as that method worked like a dream with our daughter (now 2.5 yo). He has been sleeping in the snoo after falling asleep with either myself or his mom and snuggling/snacking.

Last night was night 1 and he cried for 45 minutes before we called it as he was just crying harder and harder.

Tonight, night 2, we went about an hour but noticed that he kept falling asleep just for a second and then he would wake himself up, usually by moving his arms, and start crying harder.

In the snoo his arms are loosely contained in the sack so he’s not used to having his arms out. However we don’t really have any way to get him used to it beyond just sleep training him as he is rapidly outgrowing the snoo and doesn’t have room in there to move his arms around if they’re free, so he just won’t sleep if his arms are out.

Anyway, do we just keep on and hope that he’ll get used to having his arms out and fall asleep to stay asleep? Or is there something else that we should be doing? Has anyone else experienced this sleep and cry pattern when doing CIO or FIO?

Thank you!

(Also yes I know for real full extinction we should just leave him in there until he falls all the way asleep, but our own schedules and our daughter’s schedule don’t really let us leave him in there for a completely undefined period of time to fall asleep)