Cable shouldn't prevent from setting 60Hz. (Later on I could have problems with picture when using low quality cable, I know). I tried four cables, different settings but finally I gave up. I will use computer for 60fps videos and RPis for other, lower fps videos and movies.
Hopefully next version of LibreELEC will have 2160p/60Hz as standard (at least for RPi5) and tweaking Config.txt will not be required.
Thanks for your help, guys.
I have both Pi4 and Pi 5 running 4k 60hz.