I'm a lifelong JRPG fan who's played just about every 16 and 32 bit RPG there is. Massive fan of anything Square, can tell you just about anything about the first 12 Final Fantasy games, etc. I've even beaten some really obscure shit like Paladin's Quest. But one series I never played was SMT.
First off, the only way to play almost all of the games in English is fan translations, and I tend to shy away from that sort of thing because they're often crappy. So if there are any really good translations out there, that'd help.
Second, they had a reputation for being grindy. I can handle a certain amount of grinding. Dragon Quest games are about where my limit is. I like DQ for the most part, but I don't like walking back and forth just outside of town, mindlessly mashing "A" for hours, and most of them devolve into just that around the halfway mark. I haven't played anything newer than 5 though, so that may have changed over time. Basically, if SMT is any less grindy than the early DQ games, I'm good to go.
Third, I don't really know where to start. I've got forty years of content to sift through. Do I play the DDS games? Do they hold up? I'd be playing the SNES remakes, almost certainly. How about the side games like Nine and If?
Oh, and for what it's worth, I didn't like Persona 4 at all. The level design was insanely bland (nothing but identical hallways) and the battles were too easy. But I've heard P4 is the least SMT-like of the Persona games, so that might not be a problem.
Persona 5 Royal might be a good place to start. It's the easiest one to get into and it doesnt have the bland hallway problem of P4 because the dungeons arent randomly generated anymore. That should give you a taste of the franchise without going too hard into it to see if you even like that kind of game. Then you can go on into mainline SMT with SMT 3 4 and 5. Just keep in mind that most mainline SMT games are not as story focused as Persona is and instead focus more on the gameplay and battles and the post apocalyptic atmosphere. Also mainline SMT games are often brutally hard.
Also if you are looking for another JRPG franchise to sink your teeth into try the Trails franchise from Falcom.
I don't mind hard as long as the difficulty doesn't come from sudden, massive spikes in level requirements. There's a difference between "grinding will make this boss easier" and "unless you grind, the trash mobs across this random bridge will instantly destroy you". The former is fine, the latter is not.
So what exactly is the difference between Persona and SMT? From what I can tell Persona is a lot more story-focused, but is that it? Is it like a FF vs. DQ kind of situation where they're basically the same but with some key differences?
You're better off comparing with Pokemon. You capture and make a party of demons with various skills. As far as the Persona vs. SMT difference:
SMT: 1 hero, who is an active battler himself, and a party of captured Demons.
Persona: A party of allies, each with one Demon/Persona bound to them, which (after Persona 3) grow in strength as you deepen your bond with your allies. The main hero is a "Wild Card-" while they do have a default Persona, they are able to switch their Persona in battle. In 3 and 4, demons can be more of an item drop, but 5 brought back SMT's negotiating system since the enemies were the Demons themselves rather than generic Shadows in 3 and 4.
More recent entries in Persona focus on the social sim aspect and the story along with dungeon crawling all with equal focus. Smt on the other hand, focuses pirmarily on dungeon crawling with much less story. Thats not to say smt has no story but story in smt games often takes a back seat for long stretches of time in favor of the gameplay aspect whereas persona games tend to have very long story sequences in between gameplay that ties things together. Another thing is thst often smt games have stories that are more open ended than persona games, with an alignment system (law vs chaos basically) that often changes the final stretches of the game and the endings based on your choice, with no real right or wrong, everything is morally grey. Persona games stories are more involved but they are also more set in stone where you dont have as mich player freedom to change how things play out.
As for the difficulty, smt is known for having the kind of difficulty where you are always one bad turn away from a party wipe if you arent paying attention, even in regular encounters. However most of that can be dealt with as you learn the mechanics. Instead, the main difficulty in smt games come from bosses, where you will frequently get walled by a boss until you figure the mechanics of how the boss works and how to deal with the boss's crap. Its not a level thing so much as figuring out a strategy that works on the boss.