I'm curious on the spacing. I have just acquired very similar setup - esprit 100 + zwo 1600mm (pro version). I tested it last night and stars looked reasonably ok to me but I'm very new to this. Opinions if it's good/bad spacing much appreciated. I think I can see some elongation in corners. The setup is flattener, zwo 16.5mm spacer, zwo 21mm spacer, mini filter wheel, camera. Quick exposure attached.
After contacting Skywatcher they can apparently provide thicker felt for the rings. I have exact same issue - the tube just slides through the rings no matter how tight. The washers are in correct place. Not impressed with that at all as rings are basically not ft for purpose as supplied.