There's https://wiki.debian.org/MassRebuilds - best to talk to Lucas.
There is also the "ratt - Rebuild All The Things" tool written by
Michael Stapelberg, originally for the Debian Go Packaging Team, but
works on any other non-Go packages! I am now trying it on libwebp
1.2.4-1 which I uploaded a while ago (which I didn't test with ratt
back then but thankfully didn't break anything), and it is running
great on my local machine!
"apt install ratt", and optionally "apt install dose-extra" too if it
didn't get installed, for better reverse-dependency checking (i.e.
more comprehensive list of packages to test rebuild).