392mm chainstay length seems short to me & that 5mm clearance to the seat tube is going to make fenders or ever going larger tires difficult. For comparison, even my wifes size 53 Cervelo has around 400-410ish stays.
I dunno if it matters to you but THERMLX tubing is manufactured in Ohio, USA by Vari-Wall. I can't say I've heard of the tubing your builder wants, but then again, my experience is limited.
Now would be a good time to ask about braze-on's, couplers (if you ever travel),
derailleur hanger (in the event you ever change your mind)
water bottle bosses,
rack & fender options,
etc...Don't limit your thinking to just how you intend to use this bike this summer. How might you use it 5 years from now?
In any case, what ever you think you might need/want/agree to, component selection or otherwise, get it in writing.
It's hard to go back once the paint is on. I look forward to seeing the final result.
Here is mine, from a different builder with different goals, but highly spec'd by me.
Base2's custom bike.