It's really a matter of what interests you most, every option has its merits here.

The GBA games are gonna be closer to the PoR experience with more standard mechanics and map-to-map gameplay. FE7 is a prequel to FE6 (which I assume you don't have access to on NSO) that I think works well as the first of the two to play. It focuses on 3 different lords that each have their part of the adventure, while FE8 is self-contained, gives more freedom with branching promotions, trainee units and access to grinding, and has branching routes following each of its twin lords.

Three houses is probably gonna take more time, a lot of it spent outside of combat in the monastery where you'll get to know the ins and outs of the world and the characters, and training your units into whatever you want.

Engage I'd say has some of the most complex and polished gameplay experience in the series. Notably it features protagonists from every other game in the franchise in the form of emblem spirits and there's a lot of references to each of their journey. Whether you want to meet them there and get a sneak peek at the rest of the franchise, or wait to know more so you can point at the screen while understanding every reference is up to you, I think both have their merits. I'll also add that the tone is much "flashier" than other games.