Big maps worked in FE4 because in reality those were "split in smaller rooms" with tons of action in each one.
A big map in GBA has the 50 unit limit issue but you can have smart ways to trick the game and the players:
1) The limit is 50 for each army. You can have 150 units on a map (blue- green - red).
-> This means you have more events with green allies fighting on a different area of the map.
-> Also if someone could hack it, you could make greens also be targetable and act as enemies
which would double the amount of enemy units displayed.
(then casually "paint" greens as reds and configure their turn music as appropriate)
2) Make enemy reinforcements appear not based on turns but based on area events.
Put a lot of enemies in the start of a map and since you have to kill them to progress
once you reach a specific area trigger more enemies.
Because everyone played the game once or twice and due to the lack of knowledge
everyone tried to turtle their way through
when FE4 was designed to divide your units into groups.
Like I said, the more your replay a title the more efficient you become and the more you understand how the game works.
But who playes a romhack more than once???
Most play a romhack once and at the end throw a biased and rushed critique.
For that I blame the official devs for the reason we haven't seen a proper map since 2005 (FE8).
Might as well make FE a turn based RPG with big parties like Suikoden series
and remove the map movement.
I mean, the new players that started with Awakening,
with what are they going to compare the maps on a romhack?
These maps and their difficulty are alien to them.