Per my tests and observations I came to the conclusion that in fact the cause of the random reboots is an issue with Kernel 5.10 and mt7621 HW flow offload. While I do not have any kernel panic log to share, I'm pretty confident of this finding (the only way to capture kernel ring buffer across reboots is to attach an UART and capture via serial console, something I cannot do due to the location where my main router is installed).
After a full week running SNAPSHOT r18539-f2c3875dfc with Kernel 5.10 on an Archer C6 v3.2 with HW flow offload disabled, the reboot issue went away. Notice that SW flow offload was enabled and it is working fine with kernel 5.10. However with HW flow offload enabled the device randomly reboots itself (the reboot frequency varies a lot, from a few minutes after power up to a few days).
With Kernel 5.4 and HW flow offload enabled this problem does not happen. So, before I switch back to an older build with kernel 5.4, I may give Kernel 5.10 + firewall4 + nftables a try (since I understand that the mt7621 HW acceleration is implemented as part of the firewall).