I worked on a silly proof of concept just to mess with GB Studio 4 this week and... it felt buggy, overly complex and fiddly. Things like the "tile counter" for backgrounds that when you go in and cut detail out, even if you reload assets, does not update and the only way to make it do so is to close and reopen GB Studio... which for how often you would need that would add so much wasted time to projects. Or things like the platformer mode allowing you to set the player speed command as always... except, it doesn't work in that mode. Literally. It doesn't hide the command in that mode, or tell you it won't work. I found out via a random reddit post.
I have to stick with 1.21 anyway as the later versions cut the basic functionality to do the branching conversations I need.
As far as making carts, I use a GBxCart RW and... super cheap bootleg carts (usually Pokemon...) from aliexpress which can be overwritten with it. People on the likes of etsy offer label printing but after that getting into boxes and manuals etc I've never done.
Extremely satisfying to be able to actually have your game on an actual GB cart, even just for personal use. Feels very real then.