Hi Maciej,
The migrated code and the ENV and Friefly source code are highly compatible with .NET core > 98% according to analysis tools.
Because Microsoft Visual Studio has not yet completed its work on a Visual Form Designer for .NET core - we did not complete the adjustment to .NET Core, since we don't believe anyone will buy a migration to .NET without a Visual Form Designer.
Since you have all the source code, you can perform the move to .NET Core yourself - one of our customers did that a few months ago and although it required some work, they seem to be happy about it - Maybe they can respond to this forum post.
As for the Visual Designer, the best advice I have at this point is to use two project files, for the same C# files.
One with .NET Framework - to use with the visual designer, and one with .NET Core to compile and use .NET Core 7.
Since we haven't done a .NET Core migration ourselves I don't have a lot more information to give you - other than that one of our customers has done it and as far as I know succeeded.
I recommend that you either postpone until we've done it or try to do it with the source code you have - contracting us to help is also an option, but it'll be with a significant cost
Best