I just started a games with the ottoman. After taking albania and constanople, i was able to convert to orthodox for free just by giving up to do rebel. I think the orthodox can't become the Emperor anymore. It is still worth it to become orthodox?
I quite like Orthodox Ottomans simply for the high Patriarchal Authority bonuses.
You could then convert from Orthodox to Catholicism and then take HRE Emperor.
You don't need to. Support the weaker League just enough for them to sign a white peace. With the Peace of Westphalia even a coptic Armenia could become the HRE.