> In 2018, Unity engineers discussed that they are working on porting the engine to .NET CoreCLR

Hard task, no doubt. Unity needs to throw everything at this problem. C# in general has gotten insanely fast by default. It's very much worth taking the time to upgrade/update.

Whilst we don't compare in size and api surface, it took us a few months to get off 472 and onto dotnet6. But once we were on dotnet6, moving to the LTS after that was relatively painless; usually a few hours of work.