There’s no “final fantasy universe”. Every numbered entry (or its remake) is a good entry point. It’s not that each game tends to be somewhat separate from the series, each of the games is completely separate. They don’t share characters or even worlds. Some are high fantasy, some are almost sci-fi.
I’d recommend the FF7 Remake. It’s old enough for you to grab it at a cheaper price, and new enough to have pretty accessible game mechanics.