I like the way Path of Radiance tackles supports. Gaining Support Points by just being deployed in the same chapter. I get that it destroys the illusion a bit, if the characters don’t need to fight together because of this - but probably this is why I like it, since it doesn’t really affect the decisions that you have to make during the game.
Now of course it should still be an accomplishment to unlock the supports. And especially if it’s important for the Gameplay (like in Holy War, Awakening and Fates, to get children, gaiden-chapters, very high bonuses, Adjacent-Crits, Items, other events…), you should consider to make requiring them a bit harder, depending on the support-partners (Canon-Pairings or Childhood Friends being easier to unlock than supports with Strangers or former enemies, etc.) and how high the bonuses are and how many you can have per unit. Generally making the bonuses lower when you can have like 10 Supports, but maybe only 1 A- or S-Support that gives significantly more bonuses.
Unfortunately getting Supports in the GBA Games mostly ends up in grinding by letting the units stay close to each other until they get a Rank-Up. Imo that doesn’t really add much to the game. But that can be prevented by forcing the player to keep going instead of focusing on the Support Grinding. The player should mostly stay active, so he can’t get many opportunities to stand still.
I think the way I’ll try to do is to set the Supportrange to the entire map, so that I can simulate the PoR System. Pairings that are very likely to happen get a lower requirement, and those who’ll have a more difficult time together need more points. Also having a few supports but giving them good bonuses seems to be best imo. But I think that also depends on the number of supports and units you have in general. They’re there for multiple runs, but then again, you also want people to read them so…
Also Supports are a great way to give the characters a face. It’s not that important to give every character many supports or dialogue, but enough to ensure it’s fun using different armies in multiple runs and to show how the character thinks and acts. I think it’s a bit sad that GBA limits the supports to be on the battlefield, since off-battlefield gives you more options - which doesn’t mean that you can’t write whatever you want anyway.
I think it is a good decision to show the player what bonuses he/she will get when pairing up certain characters. Affinities could give you a hint though so it’s easier to figure it out. Except if every unit gives a unique bonus that is.