I think it will be fine, but there's not much to gain because it's not like doing patches to a RELEASE with freebsd-update is slow or very frequent at all. It's really only the major/minor upgrades which are slow, and by the time the next one is ready (15.1-RELEASE) I'm sure pkgbase will be very stable. So just wait a bit

I used it few months ago for my FreeBSD 14.3 just for test – the script worked without problems

upgrading from 14.3-RELEASE pkgbase is a pain though, you'll want to see the release notes on what to do (manually fetch the 15.0 bootstrap package to get the correct keys+repos, then fake the ABI env to install the packages)