Be aware though that, for both, changing the store path means changing the hash of a lot of derivatives. This means that you mostly cannot use pre-built binary packages any more and will end up building (almost) everything from source.
Which would be impractical.
Is there a way to easily install as root; or, easily meet the larger goal of building under GitLab or EC2? Either a direct route (nix-build directly) or indirect route (via remote build). Or would this wind up being a time sink?
I don’t know what gitlab is doing, but the docs for EC2 suggest sudo should be available.
In either case, macOS 10.15+ will require sudo, so there are probably two answers:
if they let you use 10.14, you may be able to get through a single-user install without sudo
otherwise, only if sudo is available or if you can move the store without causing insurmountable problems (I get the impression that this is ill-advised at best, but I haven’t tried)