I think you’ll need a 20″. I’m 5’11” & think my 18″ is a bit small so I’m looking for a 20″ to try.
I’ve just sold a large Singular Swift as I found the front end too high (new models have addressed this with a shorter fork) but its the same effective top tube as 20″ & I found the Swift OK with an in line post & 100mm stem.
On the 18″ Scandal with the 100mm stem I need the saddle a long way back on a slight lay back post but with a 400mm long post its a long way out although still below the top tube/seat tube lower weld.
Just ordered a 400mm long post with lots of layback to give it another go but think a 20″ with in line & may be 90mm stem would be spot on.
So for you I would think you need a 20″.