The concern about http over https is that a bad actor can intercept and change traffic.
If you allow self signed certificates, anyone who can MITM traffic can masquerade your site just like with http
Self signed does however stop passive fibre taps - to intercept you need to MITM.
There then the “remember this cert” option. If I visit www.selfsigned.com on a secure network, my browser remembers the certificate. If I then travel to another network with a MITM, my browser can flag up a warning. This is how SSH works.
However I’m not too concerned by SSL certificates as a centralised point - my browser trusts dozens, probably more than 100, root certificates. That’s not centralisation.