He held more during his Presidency as well. You know, when he was President and working so hard to MAKE AMERICA GREAT AGAIN!
I do think he's doing less for a couple of reasons. You're spot on with the energy and fitness, but I'd like to add to that that his team are asking him to keep them down because of his mental decline too and because of his inability to stay on topic or his constant insane rambling.
The other reason he pointed out himself yesterday when asked if he became President again he would do daily press briefings. He replied 'no, you would all be sick of me if I did that'
He's right and the wasn't joking either, I think he knows and has been told how repetitive he's getting and that maybe lessening the amount of rallies and press conferences would be a huge positive while doing more would be detrimental to his reelection campaign.