Not to hijack your thread...
I got a used AT115EDT a couple weeks ago and am setting it up for use with an ASI1600 and ZWO EFW, just like yours. (I went with the Pegasus FocusCube since I have that on my SCT)
I have both the FF and 0.8xFF/FR but prefer to use it with just the FF. (I have a 600mm f/4 lens)
My question is, does the imaging train seem slightly loose to you. Mine seems to have just a slight bit of lateral movement, when the camera is pushed gently by hand, that appears to be from the 2" accessory adapter.
The focuser assembly is solid, even without the rotation screw tightened down.
It doesn't seem to be from the FF being 2" slip in vs. screw on like the reducer/flatener, but that the entire 2" accessory adapter assembly is flexing just slightly even when it's rotation screw is tight.
It could also be that I'm just OCD, since this is my first real refractor.
I added the ZWO camera clamp to the Nikon lens trains because the ZWO Nikon adapter is REALLY loose. The camera would actually move under its own weight as the mount rotated. With that addition, there is no movement.
I was going to add the clamp to this setup as well, but there are issues with having the camera needing to be able to move for focusing (It doesn't with the Nikon lens) and also the support bar being in the way of the EFW/FocusCube during framing rotation.