TL;DR: IPv6's "helpful" auto-configuration fights against manual configuration at every level. The only way to win is overwhelming force: disable it in the kernel, disable it in dhcpcd, disable it on the network, flush manually, sacrifice a chicken, reboot, and hope.
My solution
Why IPv4 is "Easy" and IPv6 is "Hard". I just got my Unifi UDM pro to handle IPv6 failover from T-Mobile to AT&T and Starlink. Had to go full ULA on all VLANs, Create NAT Masquerade IPv6 for each ISP and then finally configure the ULA addresses on each VLAN.
I am going to write up and publish my solution just in case others have backup ISP and have to use IPv6 networking .
Lessons Learned
IPv6 was designed by network architects who prioritized:
Automatic configuration (good for home users)
Redundancy (good for reliability)
Privacy (good for security)
But they forgot about
Sysadmins who want predictable, static configurations
Servers that need one known address
Firewalls that need deterministic rules
