Useful 1.25 reducer and checking the general field

[Artik]

I started doing some EAA. I have AZ GTi in AZ mode, a simple 60/400 - F/6.6 archromatic refractor and sv105 camera (5.76mmx3.24mm sensor size).

I know the setup is very modest but it get some nice images of Orion etc. 

I also have a simple 1.25" x0.5 doublet reducer from SVBony (actually x0.7 due to distance from sensor) however it introduces significant coma that makes it useless for imaging since extra field of view I get is full of coma (see images below)

Questions:

1. Is there any useful 1.25 reducer/field-flattener that actually works? I head there is one from GSO but it is also simple doublet design - so probably wouldn't be any better
2. Without reducer the image is actually sharp and flat, stars are nice and round. I plan to upgrade in future to ASI 178MC (7.4mmx5.0mm) without reducer and I wonder if this sharpness will be preserved - finally it is simple achromat. I can't expect flat field over large field of view. On the other hand the center should be ok, so I wonder Am I going to get "coma" on edges or 7.4mm sensors is small enough for that case or it would be wise to get a smaller sensor since the outer field will be less good ? Is there any way to know before I buy?

Thanks

Without reducer

With x0.7 reducer