Ross was really, really good in August and September. I think that's an easy call. He should be the favorite to be the #5 starter, and at 26, he might just now be coming into his best years.
From 8/2 on, which includes 1 terrible start against the Mets where he gave up 7 ER, he pitched 39 1/3 innings to the tune of a 2.75 ERA. He looked like he was back fully healthy from the TJ surgery. It takes time to get your stuff and command fully back. I'm bullish on Joe for 2020.
I wish I could be so optimistic, but I'm inclined to come down on the side you do after a little more thought. A 2.75 ERA doesn't match up well with the number of runners he was allowing. I like Ross. I just don't see him as being likely to be a viable rotation option on more than a fill-in basis.
