I just realized that I had read another thread regarding this issue, but ignored it. This thread is about a 1.5 minute delay on startup, caused exactly by the same reason as mine: dev-dri-card0.device and dev-dri-renderD128.device. What I had done by “changing to 720p in dietpi-config” was me changing from VC4-KMS to what (I assume) is the framebuffer. The issue with those 2 devices persisted, but it didn’t seem to affect Xserver or whatever. When I followed your solution by masking those 2 devices, the postboot prompt on the local display was completely gone and it booted in a few seconds! But the problem is that I think those devices are used for 3D acceleration, which I may or may not need.
I also wanna add that using the VC4-FKMS option does display fine, but has the same issue with those 2 devices (thus, long boot time aswell). I’m kinda confused on was said on that thread about LightDM, should I change/downgrade it? I’ll probably keep those devices disabled for now, so that atleast it boots fast lol.