New toy. I bought an Epilogue GB Operator. It reads Gameboy and GBA games and save files from a cart, let's me play on Windows or Linux, then save my progress back to the cart.

It can also apply colour to original Gameboy games, rewind time, upscale the image, etc. Took a little effort to get it working properly on Linux (what doesn't) but it seems pretty good on the first evening.

Plus if you have fake knock off carts you can write homebrew games to them. Maybe some day I will finish developing a GB game and do just that.