I normally find turn-based combat really boring but I find Shin Megami Tensei games so engrossing in part due to the combat. You need to pay attention to mechanics and properly prepare for each boss fight if you want to overcome them. It's not about just gathering enough exp. Do you know any games similar to SMT when it comes to gameplay?
SMT and its spin-offs like the Persona series are known for turn-based combat systems that emphasize strategy, exploiting enemy weaknesses, and team composition. Some suggestions:
These RPGs vary in their execution, but might give you a similar satisfaction to what you've found in the SMT series.
Just to add my opinion on this list:
The Etrian odyssey series is uniquely ds/3ds style gameplay with the touchscreen map that you must make yourself. Highly recommend.
Divinity: Original Sin 2 does turn based tactics better than any game I've ever played, and I've played so many. If you like heavy world building rpgs and turn based battles that are as developed as they come, you owe it to yourself to grab this one on sale. Also, d:os2 is the precursor game and engine that eventually spawned Baldurs gate 3. So there's that.
Bravely default and bravely second are both lightning in a bottle mixes of story telling and turn based combat. They are spiritual successors to the older final fantasy games by the same studio. The class/subclass system with the brave/default mechanic is absolutely sublime and unlike any other turn based game out there. Plus the story itself is amazing. Definitely worth picking up.
OK, that's all I wanted to say.
I've played Persona 4 and 5 and am really looking forward to P3 Reloaded but it's not quite the same enjoyment I found in those games as in the mainline SMT games. Party composition is not as much of a thing in Persona. I've seen Etrian Odyssey mentioned a few times before in the Megaten fandom so it seems like something I'd be into. The mainline games I've played are Nocturne, SMTIV and SMTV. I'm definitely playing Strange Journey next and then maybe Digital Devil Saga.
Nocturne is my jam. I loved a review that called it something of a isolating, lonely and claustrophobic feeling of a game. And indeed when you are walking around the curved corridors of the kalpas, you feel like you are indeed deep deep below the earth.
Since you've played P4, Persona Q might be a good on-ramp into Etrian Odyssey. It's essentially EO's gameplay with personas, characters and skills/spells from P3 and P4. Q2 has the P5 characters as well but only the Japanese voices (which is why I never got around to it).