Big hugs, toddler NWs on top of baby ones are exhausting.
It sounds like you are doing sensible things- he goes to sleep independently, you're trying a later BT.
How do you respond in the night? My 3yo recently went through a reoccurrence of NWs. I made the mistake on night 1 of telling him a story then we had a week of him waking asking for a story again at 2am! I stopped doing stories and instead would just very neutrally say it was still sleep time, tuck him back into bed but no other chat and leaving room & repeating until he did go back to sleep I.e. Neither being super nice, nor pleading, nor shouting- just being very calm,matter of fact and, most of all, boring!
The other thing that has helped is a Gro-clock, so he can see "it's still stars" I.e. Nighttime. Hit also transpired that he's de eloped a fear of the dark so the light on the Gro-clock and leaving his door ajar has also helped.
Just some suggestions to try?
HTH
Xxx