is kludging NAT for IPv6 not a better solution versus ULA addresses?

There are very few hosts that allow only ipv6 (though there are many who only do ipv4). Ipv6 would improve internet stability and long-term communication when you’re not using a nat but that isn’t what you’re trying to build.

Or is the clear answer just use IPv6 as intended and let the devices handle their privacy with IPv6 privacy extensions?

It’s my clear answer at least.

You can also use ULA addresses for now and later add GUA ipv6 addresses. ULAs are meant to be used when you only have a dynamic ipv6 prefix so that internal devices can have ipv6 internet (GUA) while also having a static ipv6 address(ULA).