Can't use math mat expressions without ToMat function returning the result
I know I can forward the OpenCV Nuget to get this command but I think users wouldn't be expecting that to be necessary because you have forwarded the * / + and - expression nodes already

Could you also forward the ToMat node so the results of the expressions can be obtained?
(I suppose this setup could be done by wrapping the mats to the CVImage type, but on the other hand maybe they performfaster if the matexpr's are combined and pass ToMat just once.

Mat and MatExpr are something that has been discussed a few times, sadly some of the type inference/equivalence used by OpenCVSharp is not working in VL (things like Mat to InputArray implicit conversions for example). MatExpr kind of falls in this realm.
Yes it would be best performance-wise to be able to use MatExpr directly but I am afrad for the time being I do not have the time to redesign this.