The S5 is a Great bike and one of the fastest around. Cannondale SystemSix would be in the same conversation.
For the S5, I advice going with a 380mm bar, since those are wider than the number shows. Also, it is a nice option to go with 0 offset seatpost, to move the ride in a little more of an aggressive position.
For flat crits, the 48T would be too small for me personally, and I don’t see an advantage in going 1x on a road bike, but this is your decision to make.
Don’t go with too fat tires. 23c almost always the fasest option, with 25c a close second. I don’t know a single set of wheels, that is fastest with 28c.
Also, don’t look too much into weight, it really doesn’t matter.
This is my ride for crits and flat races:
