Reynolds make some double-butted Ti, but it's very expensive and high-zoot stuff:

https://www.reynoldstechnology.biz/m...t-3-2point5ti/

I think most people use plain gauge. I saw a video about the Moots factory and they were buying tubes in 20 foot lengths, so those obviously can't be butted at all.

If I was going to make your frame I would use Reynolds 853 DZB (which is steel, triple-butted, and strong AF). I'm not convinced by gussets; I think it's better to use triple butted tubes. Yes agree HT/DT is probably the weak spot. Also HT/TT.

Street trials are on topic because how you're going to use the frame affects the design!