I guess you should buy a dedicated flat field scope. And even then, you will see image errors with the ASI.
Unfortunately, most of the telescope/reducer systems were calculated for old noisy sensors with large pixels...
A telescope with a suitable flat field for new cmos cameras will cost money.
For refractors, use quadruplet/quintuplet systems, with a triplet as lens and a one, two or three lens flattener build in.
The flattener is then calculated for the lens in front of it, and that makes some difference.
For newtonian reflectors, there are correctors which can work (GPU corrector for newtonians).
For Schmidt-Cassegrains, i would advise to use an edge hd from Celestron. Unfortunately, the reducer that celestron makes will produce intolerable image errors because it is also old, uses cheap glass and has problems with the spherochromatism from itself and the sct correctors. The native sct unfortunately has only f10.
Rc telescopes have faster aperture than sct work but they need intense collimation.
Similarly, the skywatcher mak newt needs an intense collimation effort. There are people who basically screw it apart and fix some things with the primary and secondary, before collimating with cats eye tools to get better images.
Finally, there are hyperbolic astrographs from takahashi. They appear to work. And then there is the Celestron rasa. It needs a tilter but then it works at f2.2. Sadly, the rasa does not like filter wheels...
