Hi! Thank you for your nice comment! I can work on supporting cartridge rumble, I already know why it doesn't work. In mGBA and the miyoo mini, the rumble does work, as you noted, because Skyland supports GameCube controller rumble when played on a gameboy player attachment for the GC, which mgba emulates.
Unfortunately I don't have an EZ-Flash cartridge, although I do have an insideGadgets rumble cart. It takes forever to flash code to it, which is the main reason I haven't added support for it yet -- my method of working on projects usually involves making small incremental tweaks until I like the result, and it takes 10 minutes to flash this cartridge, which kills my motivation to do it :) I will put it on the todo list! To be honest, the existing rumble support could use some more work. A couple of years ago, when I first added rumble support, I just made the controller rumble whenever the screen shakes visually in the game, but I could work out something much nicer feeling if I find some time to do it.