i was comparing tart to the palaces because tart is the main story place. Mementos is, for the most part, optional, until the end where you can speed through it.
Most sidequests and nearly every social link are locked behind Mementos progression so I'd say it's technically optional but the most efficient use of it is to try and grind as much of it as you can in a single day.
Palaces + Mementos was entirely too much I feel and puts way too much weight onto the combat side of the persona formula.
Tartarus is a lot better IMO because it's much simpler, much better gated, and much faster to get through. If P3 had huge, gigantic palaces as the "end of the month" events instead of extremely short dungeons with specific bosses then it would probably also feel like too much.
Tartarus also had a lot more minibosses whereas Mementos took the route of just having them be normal enemies/personas with a possible small variation.
P5 dungeons did the same thing for the most part - which is why the theming felt so half assed for me. In the first dungeon (a castle) you would ambush a huge walking suit of armor but the battle would be vs 2 faeries and a nekomata. I'm not sure why they stopped having specific dedicated enemies but it made the combat even more humdrum because you almost always knew what enemies were weak to (because you have them as personas).
P4 dungeons aren't pinnacles of game design, but the length and depth felt way better than P3 or P5 did - with a much better story hook on top of it. I never felt like "Holy fuck when is this going to be over" in a P4 dungeon where as in P5 it felt that way nearly every single time.
i think P5 did more gameplay wise than just combat stuff, things like ability bonuses from Social links (were those in P4 vanilla?) getting to choose inheritance abilities from personas, the lil sidequests and side stories that unfurl throughout the game, like the girl stalking the guy at school. while you may feel the dungeons went on too long, i do think they're easily a step above say, the P4 dungeons, which were just "we reskinned a buncha hallways to match the theme, now find some stairs."
P4Vanilla had ability bonuses from Social Links - although limited to party members (also Rank 1 was "will take a mortal blow for you" in P4V which got moved to Rank 9 in every other version).
P4G was when you could directly choose inheritance from fusion.
It's fair to say that P4 was "too little" and P5 was "too much" so hopefully they can strike a balance with P3R and the eventual P6.
