Fortunately here in North America we just need fixed channels 2-36 for Digital ATSC but in supporting NextPVR i certainly see the mess in DVB world, and it varies greatly by distro too.
I haven't heard back from sub (the author) but I have been able to hack the binary, which is not open source and substitute /storage/usdvb for /usr/share/dvb and unzip a dtv-scanning folder there. Scanning was then possible and I can play LiveTV in Kodi and watch externally via ffplay so I know digital playback is possible on the RPi4 class. For my purposes that is probably good enough so I don't need to try that mount command. I probably would have just re-squashed SYSTEM and added it there if I knew the command line.
I would be grateful if you could package it is as addon I and would be more than willing to provide feedback and help testing on many platforms (wishing for an H2+ though). On some platforms Direct play in Kodi etc will be ok, but the biggest challenge is the ffmpeg requirement for creating HLS file for streaming to a web browser. There will be need to be custom patches for various platform. For devices that can't transcode h264 fast enough, I couldn't get 1x on the RPi4 with h264_v4l2m2m, it will only be able to remux IPTV. Feel free to open a conversation on this when you have time. I can share the hacked module with you if it will help.
Martin