Upgrading to .net 8.0 is probably trivial. Just change the target framework in the .csproj of MGUI.Core:

Before: <TargetFramework>net6.0-windows</TargetFramework>
After: <TargetFramework>net8.0-windows</TargetFramework>

And change MGUI.Shared:

Before: <TargetFramework>net6.0</TargetFramework>
After: <TargetFramework>net8.0</TargetFramework>

I doubt it would break anything.

As for the latest MonoGame, you can try updating the nuget packages. But one thing you should be aware of is that I was intentionally targeting an older version of MonoGame.Content.Builder.Task (version 3.8.0.1641) rather than the latest version (3.8.1.303), because there was a bug in version 3.8.1.303 that caused text to render with incorrect letter spacing. I'm unsure of if that bug has been fixed by now (it was like 1.5 years ago when I encountered it). A user on the monogame forums put together a useful .gif that shows the differences we were experiencing in text rendering between the different versions:

So it might not be a good idea to update MonoGame.Content.Builder.Task if that bug still persists. The other monogame nuget packages probably wouldn't cause problems when updating.