The AC88Us should work fine as wired nodes. If they do act weird though, you can set them up as their own separate AiMesh config with the same SSID. Set one up in AP mode and pair the other AC88s to it.
Other than Smart Connect not really working and AiMesh not taking the 5gz-2 band as a hidden name_dwb ssid, I haven't had any speed issues with mine on the stock firmware.
I just plugged in the SFP and it just worked, but I can't get a screenshot of it because while it used to be running my network, the AX89X is now just an AiMesh node out in my garage connected via normal ethernet. It was too unstable and I swapped it out for an RT-AX86U.
I was using my AX89X with a 1GB SFP module (D-Link DEM-310GT to be precise) connected to a D-Link switch with the same SFP module on the other end, it worked fine.
I have an AX11000 and AiMeshed off of it I have a wireless GT-AC5300, a wired AX82U (gundam), and a pair of AX92U one wired and one wireless. They've all been fine.
The only ones that have given me issues is an overall cranky RT-AC5300 and the AX89X