In the Roon universe, a classical composer is indeed not allowed to perform own works. Therefore, the “Composed by” will never show albums where the classical composer took the liberty to conduct or otherwise perform own compositions. It was explained somewhere why the decision was made, I believe to keep out low (metadata) quality budget compilations which often use the composer als a performer credit. And that is also why your approach to tagging won’t work well with Roon. Maybe Roon tags (ROONALBUMTAG, ROONTRACKTAG) can help you with getting things sorted the way you want it.