The mods I've done with my GBA (AGB-001):
1. IPS v3 mod.
I have tried almost every kind of backlit screens and this is by far the best one for me. It's a fully laminated LCD, so there will be almost no backlight blead, or dust into the screen.
The image will also be clearer because the reflection and refraction of the screen is reduced. The resolution of this IPS screen is higher than the original one from GBA, but it provides several filter effect so the "GBA native resolution" mode can be emulated, at least it's perfect for me.
But this screen can only be fitted into the special shell, make sure to get the correct shell with the screen together.
2. Lithium battery mod. There are actually two options, the first one is to grab some rechargeable 1.5v lithium AA batteries, the advantages of it are: easy to replace, no mod requires, long battery life (around 10 hrs with EZO). But the downside is that the voltage of these batteries will maintain at 1.5v even if there is only 10% power left, so the power indicator on GBA will not turn into red.
The second one is to find a USB Type-C charged mod battery pack, it's more expensive, even longer battery life (more than 12hrs) but you can no longer use any AA battery.
3. Audio ampifier. It's in my todo list, to make the sound from the speaker louder and clearer.