Short sleep

I have struggled for quite some time with getting my baby to sleep on his own (I’ve posted questions before). We’ve figured a lot out and made several improvements. Turns out his aversion to sleeping alone had a lot to do with his silent reflux. Sleeping on his back allows stuff to work its way up into his throat and he wiggles around and wakes himself up. Well, I’ve started putting him in this napper my sister in law gave us that keeps him elevated and calms his reflux a little (approved by his pediatrician). It’s made a huge difference and I can get him to sleep by himself now...but his sleep stretches are much shorter.

When sleeping with me (please no judgment, I would stay awake while he slept and just sleep when my husband would get home), he was sleeping 4-5 hours straight before waking up to eat. He’ll only sleep for 2.5-3 hours tops in the napper before waking up. Sometimes as little as 1 hour. This is a big improvement as before he wouldn’t sleep at all by himself, but it is still exhausting. It doesn’t seem like sleep regression because he’ll still sleep 4-5 hours if I let him sleep on me. Sometimes I think he just isn’t as warm and cozy, but I dress him in a long sleeve onesie, socks, a fleece sleeper, and a fleece sleep sack. But he definitely likes to be warm.

Any ideas? He was 6.5 weeks premature so I know all night sleep is going to take longer but it’s frustrating that I know he can sleep a longer stretch and just won’t when he’s sleeping on his own. I also know we’re coming up on the age where he’s going to make sleep associations so I don’t want him to rely on sleeping on me (perhaps it’s too late for that, I don’t know)