I have had first-hand experience of these Xorg crashes and lockups in combination with either sleep or un-powering monitors.
I would 100% agree with Hermann that this is highly likely to be improved with kernel update. In short, I would go to *at least* the buster-backports kernel i.e. https://backports.debian.org/Instructions/ apt-get -t buster-backports install linux-image-amd64 linux-headers-amd64 Over the recent 5.10 debian kernel provisions, I have seen incremental improvement in this issue, to the point that now this seems to have gone away. I still experience complex MST monitor arrangement reordering the displayport- virtual numbers (fixed with xrandr script) but NO LONGER crashes needing reboot or restart-X11 like before... In any case, please report back how 5.10 series kernel does/does-not avoid the issue occuring for you. --Simon p.s. The buster-backports kernel [5.10.0-0.bpo.5-amd64] is lagging behind a little. Hopefully .bpo.6 and .bpo.7 will come out soon. I notice debian-deriv MXlinux have put out their own 5.10.0-7mx kernel already.