I don't think there is a good term in the context of the Internet. Or offline for that matter. Even if you printed your blog and handed it out on street corners, for it to be "fully owned" would that mean you make your own ink and paper?
"Vendor-agnostic" maybe? It's not perfect but I get where OP is coming from. Personally, the use of the term is completely reasonable.
It would mean you owned the ink and paper.
The OP's fully owned is analogous to someone else doing the printing for the privelege of spying on your readers.
You're being obtuse. Paper and ink are fungible commodities. Try again.
So I agree in the sense that, you're always going to rely on something. Even if you're hosting on hardware you own at your house, using your own self-signed SSL certificates, you're still relying on an internet connection from some company.
But, I think using the term "fully-owned" to refer to pushing up to GitHub, then deploying to Cloudflare Pages is definitely not "fully-owned"