It has been a while, but I've learned a lot about IPv6 since 5 months ago.
To recap - you helped me get IPv6 set up on my Mikrotik router, but Frontier only gives me a /64. In this comment you recommended looking into NPT to help subnet my LAN.
I think this is basically what I'm doing, but I'll do some more reading about it. Right now I'm using IPv6 Firewall -> NAT -> some masquerade rules for the ULA addresses I have set up.
Like you said, they don't really reach the outside world as IPv6, since IPv4 takes priority over them.
So just to clarify, you're saying if I assign a random, unallocated GUA in the same fashion, and then NAT it to the prefix from my ISP, that could get around the browser limitation?