This is pretty much like every other porting project where you have an old neglected project that you want to bring up to date. Maybe there is an easier way but I don't know what it is. Staying on the legacy .NET Framework isn't the end of the world, you're just committing to maintaining it as is. If you want new features as they become available, well... that's on you, don't wait.
Maybe you move some functionality out of your legacy GUI app to microservices written in .NET 5, parts that are unlikely to have any porting issues. In any case it's not like anyone is going to be surprised by what's going on. Maybe the community will back port some new things to .NET 4.8. MS will be devoting their efforts to enhancing the new platform while keeping the old one on life support. I think that's reasonable. (edit: typos)