Make a SSS dummy, rating your rotation from F to SSS, similar to Devil May Cry combo rating system.

Allow the dummy to show two scores, solo and group.

Solo = rating your solo rotation.
Group = rating your rotation based on current group synergy.

It surely would take some dev time, but it's needed. Let's start with that, and if it works fine we can put this for every boss.

Scores updates for every server tick.

I think this works fine for self improvement. If someone asks for screenshot your score, simply join other group or create yours. This feature is not guilty, the comunity is, and it's up to you to report that.