Sleep regression? Separation anxiety? Teething? At a loss with my almost 11 month olds sleep

About a month ago we started to experience some sleep issues at bedtime with my previously great sleeper. He is roughly 10.5, almost 11 months and not sleep trained. Before, I’d put him down fully asleep (nurse to sleep), and he’d sleep through the night or have one wake. I could even sometimes put him down awake and he’d fall asleep on his own. He takes his naps independently in his crib usually fine during the day. However within the last month, naps have stayed okay, but he will wake a couple hours after I put him down just screaming and crying, and it will take rocking and usually at least an hour or two and a couple attempts to get him back down for good. He had a rough couple weeks, then a really good week, and right back to it last night. It took 2 attempts to get him back to sleep, and even 30 mins after that transfer, he woke crying but fell back asleep within a couple mins (I try to give him 5 mins when he starts to cry bc he can calm himself down, unless he’s clearly escalating).

I want to be able to put him down awake so the first wake up in between sleep cycles isn’t so jarring for him, but I worry it’s not a good time to start sleep training. He seems to have bad separation anxiety, and I’ve no clue if he’s teething. He has no teeth yet, so idk what to look for. I guess I’m here to ask, when did you know it was a good time to start? Is it bad to start during a regression?

Our day looks like this: 3/3.5/3.5-3.75 max
DWT of 6:30-6:45 unless he has a rough night then we let him sleep til 7:30ish
Bedtime routine: bath, book, nurse to sleep

I’m also planning to wean around 12 months so that’s why I hope to be able to put him down awake. We did CIO actually at the beginning of this regression, and I hated it so much. It worked. But I went back to nursing to sleep bc I had a lot of regret around it. I’d prefer to not do it again. But the constant screaming wake ups and rocking him to sleep are getting really tiring and it’s all really puzzling too, especially since he can fall asleep on his own for naps! So I just don’t know what’s happening. I’d appreciate any advice. TIA!