Just looked into this, didn’t realize everdrives were Ukraine based. I will look into the EZ Flash jr for the time being as well as emulation. Hopefully Krikkz can get his family and team out of the country soon, no rush on my end. What they are dealing with goes beyond video games.
Krikzz posted on twitter that him and his family are safely in Spain now.
twitter.com/krikzz/status/1503753707346927626
I have two GB Everdrives and they're both excellent, so if you can find existing stock or a used one I highly recommend them.
Also, since you mentioned playing on GBA, keep in mind the difference between flashcarts for the original Gameboy that run GB/GBC games natively, and flashcards for Gameboy Advance that can emulate GB/GBC games on the GBA.
The other alternative worth mentioning is the El Cheapo SD (that's what it's called). And there's something called the USB 64M, but that's much older and doesn't have much space (and is probably not so easy to find anymore).
I have a USB 64M and can confirm they're very basic compared to modern microSD based flashcarts. Might be worth it if you find one very cheap.
Another option is inside gadgets, but they're mostly good if you want very specific hardware features (like rumble or rare mappers) and they also seem to be suffering from chip shortage a lot lately.