The MAX & NG have the same lift & fuel capacity, both are now ETOPS, the max is of course heavier and it's a Cat 'D' aircraft for minima.
The only real benefit of the max is lower fuel burns saving at best 250kg per hour so even on a 6 hour flight it's only 1500kg
On OTS routes you'll normally be capped at 340 going west and then climb to 380 after the exit point so around 165 -170 is the practical pax limit, east bound isn't an issue, but either way I suspect that the A321NeoLR will be more suited
Taken across all routes ( PVD BDL SWF) to/from (BFS DUB SNN ORK EDI BGO) it's doing ok, there is bound to be some tweaking, I think DUB will work year round, I'm less certain of the BFS point to point market and onward connections inbound to BFS are far less than DUB