Personal opinion, I don't understand why more parents haven't moved to use Pi-Holes for content blocking. Since you can tailor your own blocklists, it's not impossible to do, and I'm pretty sure some enterprising folks have already made a Pi-Hole block list for porn and various adult websites.
Further, if your child figures out how to bypass the block, they're learning valuable skills, if nothing else.
To be fair, it seems like you're more concerned with overall screen time, and I don't have a better solution for that than you already have.