If you have done Stormblood you can unlock Heave on High. You can find more info here: https://ffxiv.consolegameswiki.com/wiki/Heaven-on-High

I reached floor 50 solo by first running with a random party to build up my Aetherpool. Then I did the 20 to 50 solo as White Mage and it was pretty easy, just a bit time consuming.