If you just have two routers that are hard-wired you probably won't see a ton of benefits in using Aimesh over your old Tomato setup. Aimesh is beneficial because it allows you to connect two routers together via a wireless backhaul, there is centralized management, it will auto heal if you have...
All of the other devices that are Aimesh supported have indicated in their firmware release notes that Aimesh functionality was added, the 66U has never indicated that in the release notes and it is not listed as supported on the aimesh marketing page. Who cares if it once worked, you got lucky...
I think you answered your own question. You are using a device that isn't Aimesh supported and none of the firmware release notes for the 66U indicate Aimesh support.
I also use NVRAM to restore custom client list, static dhcp, and vts rule list. Pretty much everything else can be configured via the UI in about 5 minutes so not a big deal. Usually with small point releases I wouldn’t do a reset but clearly even with small releases it can be needed so I also...