A few days after we released a version for Linux only I can say that QMC2 builds on Windows again!! In the end it's an easy task
... I only tried the official MAME tools and an additional webview package:
pacman -S mingw-w64-x86_64-qt5-webview 5.15.3-1
After that, I could run
to build release/qmc2-mame.exe. So there's hope for Windows builds for the next version!