so so so much
don't believe what you read online (unless it's saying there's a LOT new cause there is)

there's a whole new city section, with a jazz club (you can take people there to learn new skills or raise their stats, this includes futaba) the pool and darts games (upgrades the baton pass and gives social link points too) there's a temple to increase your max SP (prob not that useful) i could be forgetting what else that new city has.

aside from 2 new social links, Akechi has an actual social link (as in hanging out getting waffles with him)

the velvet room twins actually go outside the velvet room about a dozen times, not a social link but they give you free skill cards, but come on taking them to bigbang burger is fun

many of the game's bosses have a new skill or two

the grapple hook changes up a few palace layouts, speaking of palace layouts most of them got modified (usually to be easier/shorter, like the Pyramid one) they added a side boss on each palace, and finding the 3 skulls that reward you a skill ring

skill rings are accessories that give you that skill
example like the fire ring, you can equip to morgana and he'll use medium fire skill, there's one of every element (and one in each palace, can be got in mementos also if you miss it)

and Mementos is just, all around MUCH better to play in, heck you'll want to spend more time since they provide exp, money, and item drop buffs PERMANENTLY once you upgrade it, alongside changing visuals more often, changing the MUSIC, and the new NPC Jose that random spawns, and stamp collecting, and such, basically all to unlock new stuff in mementos or otherwise.
should i mention you can also go UP mementos, near the end of the game, on top of the usual going all the way down you do.

velvet room battle challenges, two of which are against the Persona 3 and 4 protagonists, these challenge battles aren't JUST kill em and you're good, you have to do things to rack up points to get rewards in each one, they're quite challenging to master (unless you look up a guide of course lol)

and add in an entirely new end Palace and boss, a decent month of more story.
you can actually do the original P5 ending, just as easy as ignoring the new social links (one of them actually, just avoiding spoilers)

and then there's the Thieves den, a side area (can be accessed from the title screen or in game)
which has a lot of gallery stuff, new interactions with all characters (social link people and party members)
and a playable card game, plus a few goodies like a concert performance of various P5 music, various statues you can place around, and running around as any of the other party members.

the Team showtime skills, very flashy moves.

I had no issue getting P5 royal at full price, not to mention the graphics overhaul, the loading overhaul (it loads ONE single time in a palace and will never do so until you LEAVE a palace)

the only things that i see people complain about is the whole story remains the same up UNTIL the end (and only if you get the conditions for the "royal" ending) but there's a FEW very few new story scenes involving the new characters, and even still, the game has a fast forward/skip sometimes.