In my case, I start with all 3DS games and play Three Houses then (for Fates, I only played Birthright), so after Birthright I played Warriors which is more considered as a spin off, but it was great anyway! And then played Echoes: SoV which is my favorite FE because characters and story are well writtened in my opinion. But I don't finnish any other Fire Emblem (began a ROM of Ankoku Ryu to Hikari no Tsurugi some times ago but still not finnish it yet, but planning on play them) so I would recommend you to play maybe Warriors, I guess this game is better for casual player, and playing system changes of course comparing to the other FEs as long as it's a hack 'n' slash game.! I'm hesitating between Echoes or Warriors, even if Echoes is a great game that have some new features that other FEs don't have (at least FEs on 3DS).
Hope that helps! 😄