3-4am wake ups

My son is almost 9 months old and was sleep trained at 6 months, then regressed and re-trained less successfully a month ago. He goes to sleep independently (still cries but I understand that’s ok) but wakes once a night, usually around 3-4am. If we try to let him cry he quickly starts screaming like a banshee. We usually wait 15-30 minutes and then eventually just go to him, which I realize might be solidifying the habit. Sometimes we’re able to resettle him by rocking or patting, but often end up co sleeping the rest of the night. It sometimes takes him up to 45 minutes but he always goes back to sleep and wakes up between 6-7am.

His schedule is 3/3-3.5/4 and he naps between 2-3.5 hours. More or less nap time within that range doesn’t seem to change anything. He goes to bed between 7-8pm. He’s night weaned and appears to just want comfort.

How can we get rid of these wake ups? Is letting him cry our only option or could his schedule be causing this? He doesn’t respond well to Ferber check ins.