I've used MDM to make my iPhone dumb. It's great! I wish there was an easy way to publish my configuration so others could use it / adapt it, but it's a little involved because you have to wipe your phone the first time you set it up with configurator.
Supervised devices cannot be backed up and restored to supervised devices. You can only backup/restore to an unsupervised target. If you want supervision, you have to abandon your backup.
It sucks.
I’m curious about this, what did you do to make it dumb?
I did that when I had an iPhone, I agree it was excellent. More people should do it.
Looking forward to MDM support on GrapheneOS.