I also did the mentor and my thoughts are:
Go dps. Preferrably a dps that can raise so RDM/SMN. It can save an entire run.
Being a mentor made me take off my titles (ultimate titles) because I got abused so horribly sometimes by just having the combo of title and crown. It honestly made me feel so angry towards the community.
I did get toxic at times in the end because I was so tired of the abuse (I think one time I asked "are you going to listen to me or would you rather just wipe?", I never insulted anyone or namecall. Granted. I didnt do it over two years, I grinded it quickly over a summer, so I was doing runs multiple times a day and that probably made it feel worse or like people were constantly being buttheads.

Once I got my mount I stopped. Not because of "I was done" but because I just didnt want to hang out with the community anymore.

In my experience most mentors went quiet. Some would try to say something but most nonmentors would either not listen, insult or just be really, really bad. There were times ofc where you did spend an hour in ifrit or whatever and clear and that was joyous. It was VERY satisfying getting a group with positive people that actually listened and then you guided them to a clear.
In regards to the "you can do this unsynced" it is just being helpful. Some people would be really thankful and we could show them how to, while others would explain they wanted the experience of the synced run, which is fine as well.