help on kwin source build

Hello,
I am trying to build kwin from git source.
I see that KUbuntu ships with version 5.18, so I switched to branch
Plasma/5.18
I installed all kwin build-dependencies using apt.
I try to build kwin, and it tells I have a wrong version of qt: 5.12.8
instead of 5.15.0
So, I guess that git version is different from KUbuntu version, it uses a
more recent version of Qt, correct?

I also see that it's difficult to obtain Qt 5.15 pre-compiled, as the owner
distribution policy has changed, isn't it?

So, how do you suggest me to go on? should I manage to build Qt from
scratch (argh!) or should I use KUbuntu kwin version instead?

Thank you.
Luca