Mine couldn't have coped with any flexibility on sleep times when he was 2yo,. very much into routine - of course that doesn't mean we didn't have BT resistance, birthday developmental phase and the molars caused huge problems but put those aside what he really needed was consistency, a very predictable routine.
You didn't say what time he finally went to sleep at BT, I am guessing 9.30 based on the 6hr A time you mentioned. 9.30pm is certainly too late. How about cap the nap, how does he do with capped naps? So many LOs adapt well to it, 1hr, of if this is already common reduce to 45 min or 30 min. I'd really try to keep BT no later than 8pm but that's my personal limit on BT. I know when mine went through the 1-0 his nights went low, really low, like 9 hrs or less. A 10 hr night with the nap in place is probably not too bad really until he's ready to drop the nap.
FWIW mine pretty much thought he was ready to drop the nap at 24/25 months but he wasn't, it took him a while of nap resistance and/BT resistance to finally realise that he wasn't that happy and he then went back to accepting nap and night time well for a few months. So, this blip around the birthday, it is quite normal and can look like the 1-0 drop but the final nap drop might not happen for another 6 months.
Sorry there's no clear answer - I'd try capping the nap shorter as it works for mos even if not mine.