Asleby, let me tell you that this was our situation this past week to a T. My LO was doing a few long naps at 2 hr 25 min A time and all of a sudden, the naps started to get shorter. Someone advised me that a 1 hour nap was likely UT, so I bumped up the A time to 2 hr 30 min. Then we got a 40 min, presumably UT nap (and it was hard to tell if OT or UT because he was sort of crying, but not intensely). So today I decided to go with 5 more min hoping that was true....
And by golly, today we got a 1.5 hr nap on A time of 2 hr 35 min, and the 2nd A was 2 hr 40 min with a nap of 2 hr 15 min! That has never ever happened before
All this to say, that I have a hunch you might need to lean a little towards the higher A time, especially if he is chatting in the crib happily - perhaps you could try 2 hr 35-40 min again to see how it pans out?
I know you are so close! I was in your shoes just a few days ago and I can testify that miracles can happen to you too