I'm not a big fan of Get Blue Spheres, really never have been even back in old Sonic 3 but at least in there it was more bearable as just being a few special stages. I agree that having them at every checkpoint in this game and the sheer number of stages there are gets exhausting pretty quickly.
The bigger issue I personally have with it, though, is that while it's nice to have for unlocking fun little secrets and extras, it does a very poor job of working as an actual bonus stage within the context of the game as you're playing it. You can't get lives or shields or anything from it that will actually help you in the stage. Hell, even just allowing you to keep the rings you collected would be a help and provide some actual incentive for playing them for getting extra lives. If you're playing and aren't interested in unlocking outside stuff you can't even use outside of No Save runs...there's no reason to ever do it.
If there's ever a Mania 2, I hope that the checkpoint stages are built differently to be 1. Quick (lemme get in and out in 20~30 seconds at most) and 2. Providing something to actually help you in the moment. If they raise the ring requirements to activate it as a way to balance things out, then so be it.