No, I don't. I'm investigating if it's doable. At this point, I have a working server with php7.4-FPM.
The legacy app expects "phpBB 3.0.4: PHP 4.3.3+ (>=4.3.3, >4.4.x, >5.x.x, >6.0-dev (compatible))".
I don't know the difference between the "php" package and the "php-fpm" package, especially since I read I shouldn't install the former since it relies on Apache, and therefore installs it.
It sems rather easy: https://www.atlantic.net/vps-hosting/host-multiple-websites-with-different-php-versions-on-ubuntu-18-04-vps/