Mobile Device Management (MDM) is the only effective way to restrict idevices. All you need is a macbook and Apple Configurator. You can remove safari, blacklist or whitelist websites, block installing apps, block deleting apps. It's really customizable. Edit: expanded acronym.
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.
Looking forward to MDM support on GrapheneOS.