My Speedvagen, roughly a 52cm, built up by me when they still sold only framesets comes in at 16lbs 4oz including pedals and cages. I think that’s pretty good and I didn’t really do anything that would be real weight weenie-esque. The carbon seat tube is said to shave a few grams. Standard would be a steel seat tube.
no chance in heck that you could come in at that price point but you might want to look at their ready made disc’s. Stunning bikes where while not fully custom, still maintain all the traits of a custom Speedvagen and include having the seat tube cut to your proper measurements. I believe their disc bikes come in well below 18lbs and have the clearance you seem to be looking for.