Instead of doing /etc/apt/sources.list sorcery in the future, you could replace “testing” by “trixie” today. That way, at the point in time where “testing” becomes “stable” you do not need to edit your /etc/apt/sources.list. The updates and security mirrors also exist already for trixie but are of course empty but this means that you can also add those today already and have things set up for the future:
deb http://deb.debian.org/debian trixie main
deb http://deb.debian.org/debian trixie-updates main
deb http://security.debian.org/debian-security trixie-security main
The one point of pain may be that after the trixie release, the MNT repositories will continue to build packages for unstable which will work for a while but will break after new uploads of glibc and friends. So in the future, you might want to switch out the MNT repo for the Debian stable repositories on reform.debian.net which are currently only serving packages for bookworm and not trixie.