The generator has few patterns that hurt SG incrementality.

1. Having Compilation in the source output step (the last step) in the generator pipeline. That effectively means this step will always be running and it kinda acts as ISourceGenerator.
2. Having IMethodSymbol in the pipeline models (in MethodToGenerate)
3. Having syntax nodes in pipeline models (in MethodToGenerate)
4. Attempting to produce diagnostics in generator. While the API exists, I do believe it's discouraged, and a separate diagnostic analyzer is more preferred.
5. XmlDocumentation is unlikely to be comparable. It has Dictionary<string, string>, but if two different dictionary instances are compared, the result will be false even if they contain the same key-value pairs.

IMO, point 4 is best addressed first, as it might already simplify the implementation and make the rest easier to fix.

cc @stephentoub