I can see why they let Smith try this one... all the Speed Force doppelgangers can act "off" and blame it on not being the real thing... so there's some built-in insurance if Smith stinks it up.
I wanted to like the cosmology way more than I did... I think the impact was lost because of what happened in the last episode of 12 Monkeys and the decompressed exploration during Wally's comic book run. The single best thing I can say about it is that they seem to be confirming, resoundingly, that Barry is NOT the source of the Speed Force. If he's mainlining it like Wally did, that, to me, is enough and more mystical and sophisticated than claiming a lab accident on one Earth created a cosmological fundamental timeless force. So I love that aspect on paper but it didn't click for me in the episode.
It's also not really evident Barry got a level-up like Wally always did after a big spiritual journey. More like a return to status quo. Barry's still not stomping a brainless Girder so he's still about where he was before. You kinda want to see Barry have his Neo / Matrix moment after his Enlightenment to prove he's upgraded.
Last nitpick... Grant shakes his head side-to-side like he's saying "No, no, no." waaaay too much when he's trying to be earnest in this episode. Someone needed to call him out on that. Probably wouldn't have noticed if this ep clicked for me, but it was kinda sappy filler (the B-plot was literally zombie Girder woefully executed with really cheesy shoulder-first shuffle)... he didn't even commit to saying he'd go after Iris!