I tend to agree with JF.
In terms of ability, I think Mark was already at his peak in 1978, and he remained at this peak at least until 2005. There was no room for improvement, really, he was a god from start, the first DS album is perfect...
He just felt bored to play always the same way, so the style varied a lot throughout the years. But I'm pretty sure he would have had to ability to play in 1978 like in 1992, and vice-versa, had he wanted to.
I agree that the OES tour was too much of a show-off exercice. As if Mark was so bored and not into it that he wanted to compensate by playing crazy stuff. Take TR, for instance. The end solo is out of the world, granted, but that's not doing justice to the song. Too fast, too rocky, too much for what TR had to deliver. No feeling in vocals, no intro, atrocious pedal steel, cheesy sax... Hardly the best versions. Yes 1996 was so much better, more "back to basis" while still very skillful. But Alchemy has the best vocals ever, with skillful guitar at the same time. One the best TR of all times..
And 2019 was so unexpectedly good, because he knew what he could play and what he couldn't, and what he chose to play was outstandingly delivered. WAM, STP, MFN, even BIA for some of them were the best ever. Speedway was still great. And the feeling was still there - take the new ones, who else could manage to get a crowd sing "Have you got my bacon roll" ? And take Matchstick man. A minimalist song about his past, bringing us to tears. Mark's equivalent to Beatles' Yesterday - a great example of l"ess is more"...