I haven't complete dialed in my fit on my bike yet. I have a quick release on the seatpost that's helpful in deciding just how I want to have it. Once I get that dialed in, I'd like to get rid of the quick release (not that there are a lot of people who're going to want to steal my saddle, but still...). Once I decide I've got it set up the way I want, I can probably just pick up a bolt at the lbs right? I imagine there's a standard size (as seatpost diameter shouldn't effect the bolt as far as I can tell). Or can I just figure out the size and pick something up at the hardware store?

Go to your LBS and have them take a look at it for you.They can get what you need or they may have one in stock.

Once I get my new handlebars and brakes installed and my fit dialed in, that's where I'm headed then. Thanks.

If the collar is a part of the seat tube, you can use a new bolt and washers, and the existing nut.

If the collar is removable, you can replace it with something like this: https://www.performancebike.com/bikes..._1033972_-1___

I've done both on my bikes.

I'm pretty sure the collar is part of the bike, but I'll have to make a second check of that.

These are made to do that https://www.bikeman.com/ST4500.html
a QBP item , so your LBS can supply one ..

It's like a QR wheel skewer sized bolt , with a symmetrical
looking nut and a thing around the bolt head to match.

long enough 5mm bolt a nut and a flatwasher under the nut and bolt head
would also do.