Have you considered using a reducer/ flattener? They're not expensive and reduce the effective focal length to 1280 mm with an aperture ratio of 6.3.
With your 2.4 micron pixel dimensions, you're very over-sampled, especially considering the mount and seeing.
Another piece of advice: isolate your SCT tube. Only the tube. This reduces tube currents dramatically!
Latest Stellarmate on Rpi 4B, 294mm-pro, EFW-8, CEM70