I don't know the exact cause, but I believe the crashes/non responsive nature is due to CPU stalls, I managed to capture some logs of a few:
I've caught the system load going insanely high after about 3-4 hours of up time, which is when the non responsive state starts and believe it to be the CPU stalling out. When this starts there is a small window where the router is responsive and you can get into SSH. When I do this, if I stop mwan3, it seems to stop the issue after a while and when the system load calms down, I can start it again and then everything is normal. If you can get past this weird 3-4 hour mark, the uptime is fine after that:
In terms of my configuration, I am using VLANs as well with mwan3. I have various different network interfaces, L2TP, Wireguard, DHCP etc.
I am unsure why the CPU stalls out, but it didn't start happening on 19.07 initially, until around 19.07.4, but after reverting back to 19.07.3, I did see the same behaviour, so I wonder if it is a combination of packages/configuration. The latest change network/VLAN wise is L2TP with a new VLAN, but I can't be sure that's it.
However, it is interesting you have experienced the same behaviour. VLAN configuration could be a clue. Please let me know your specific setup and maybe it will provide further clues.
I don't believe it to be a power supply issue.