I think GBA Mana game might be only marginally ahead of Phantasy Star 3 in the "no hope, don't bother" for me in the hacking possibilities to improve stakes.
Going after text might not do much, though I will have to look (if it is a nice graphical effect it could be blanked out, if plain transparent text then replace with spaces might be a thing). Assuming you don't care to force cheat the level counter down if doing the low level thing (don't know if this is some kind of save the level up for a quick heal ploy) then I would probably go with https://www.romhacking.net/documents/361/
(again an older emulator but should be easy enough to convert to something like no$gba debug) except rather than going back to the part of the ROM there will presumably be a check on experience or something (easy enough to find for a cheat search, might even be an existing cheat out there) that you can disable the call for. Alternatively whatever fiddles with the OAM or BG to display it on screen can probably be NOPed out instead.
If it is purely a screenshot thing then do check it is not left on another layer that you can disable. Not ideal but depending upon the emulator (assuming you are using one) then it is easy enough to disable layers for a second or two.
Improving AI is a tricky task at the best of times. There is a reason why it sucks in most games. Best you can usually reasonably go in for when hacking is make the AI faster/hit harder/have a greater lock on radius/have a greater number of invincibility frames or indeed go the other way and nerf the enemies a bit. Hacking in some more advanced AI is possible for an action RPG type deal but a lot of work to do well/create a more competent setup.
Not sure how in game explanations/guides/tutorials and questionable level design you can't maybe backtrack to later are related but both those are within reason.
Changing rewards for quests is within reason, as is maybe some kind of crazy grind to unlock (passive is easier than active display new text but new text is hardly out of reason either). If EXP/kill count/money = blah then insert item into inventory is a thing.
If the content is unlocked by a flag in memory (say it is an extra quest/building/whatever) then that is also easy enough.
Re: Super Gameboy stuff.
That will depend entirely what was done for the hacks -- the supergameboy in most commercial efforts was underutilised considerably
being my usual link of choice for SGB stuff, what was done and what the potential was)
If the hackers leant into it the clawing it out becomes much like backporting DSi to DS, game gear to master system, GBC to GB, FDS to NES...
Several of those games, and many others, do however have colour hacks (often using the DX naming scheme, though pokemon can be the exception) for which many are onhttp://www.romhacking.net/?page=hacks&genre=&platform=8&game=&category=&perpage=200&order=&dir=&title=&author=&hacksearch=Go