My ISP (T-Mobile Home Internet) only provides /64, therefore no PD (prefix delegation), I can't use IPv6 with OPNsense, even though the OPNsense appliance will get an IPv6 address, nothing can be passed to the LAN clients unless I can enable NDP Proxy/Relay mode, in OPNsense, and so far, I can't. Does it make sense to fully disable IPv6 in OPNsense vs leave it half-active with no use?
You can use NAT just like you do with IPv4. It's ugly, but it works and is better than no IPv6 Internet connectivity at all.
If you don't want to do that, you can still use ULAs in the LAN(s) for local IPv6 connectivity.
And if you don't want to do that, OPNsense itself can still use its IPv6 Internet connectivity for DNS, updates, VPNs, ...
So I don't see a good reason to fully disable IPv6.
Cheers
Maurice