I only know of one FE fangame made with Game Maker and it’s the recently released Heroes Tactics
They made the game from scratch, which is probably the reason why FE fangames made with these less specialized engines are rare. One would have to make an SRPG engine from the ground up if working with something like Unity or GameMaker. So ROM hacking or working with an engine geared towards making SRPGs like SRPG Maker, Tactile, or LT-Maker would be a simpler route to go down for most people
EDIT: Completely forgot to mention the FE-inspired games Stratagem Lost (also made with Game Maker), Astral Throne (made with Unity), Those Who Rule (also made with Unity but apparently started off as a ROM hack), and Eternal Legend (made with Construct) But yeah, point still stands, you gotta work from the ground up like these guys did lol