I had not been able to get M2P on my RPi4 8GB to boot up at first either. My plan was to set it up as an LMS server only with no soundcard.
I used the advice from Bencat57 and Iwanman above and got an RPi4 4GB and used the DL of M2P on an SD card to update it to version 2.52.
I then took that SD card and put it in the RPi4 8G and it would still not boot. That was anticipated, but just thought I’d try.
Then I updated it to the latest Kernal using info I found on the internet and rusty Unix skills I haven’t used since the 90s.
All went well and the RPi4 8G has been running Squeezebox Server/LMS for over a week now without any problems. Doing a rescan looking for new and changed media files went from about 14 minutes to about 3 minutes.
I then repurposed the RPi4 4G to be used with a new HiFiBerry DAC2 HD and it is running well on M2P Beta. I had planned on getting the new RPi4 and HiFiBerry card in the future, but ended up getting them earlier than planned to help get the 8gB RPi up and running.
Thanks for the help and suggestions from the folks on this forum.