Did I brick my GBA?

Check if it’s just a battery terminal problem - anything that uses disposable batteries anyone can change is designed with reverse polarity protection if the designer has made it past basic engineering 101. If they’re bent, I would see if you can correct it, but depending on the amount of damage done add a little extra to it so there’s room to compensate for the damage. The negative terminals are spring based in the GBA and GBC and are sadly, easy to break if you abuse it the right way. That said, the reverse polarity diode can burn out with time so it doesn’t always save you but it’s usually not reverse polarity damage.

If it still doesn’t work, what I would do is find a parts GBA that HAS NOT been backlight modified so you know the board is original and replace the damaged terminals. This does require soldering, but if you can get a busted one for pennies to the dollar it’s worth taking a chance. Don’t spend nearly as much as a nice used one, so you may need to stand your ground on price depending on how stubborn the seller is.