That bike looks great, especially considering it’s your first frame!
I think there is a high likelihood that it should be fine. Wall thickness is only half the equation, the tube diameter dominates the strength equation, and 35x.09mm is beefy. I think most SS-ST failures you see are on 29mm tubes with thin .7 butts.
Total guess: I think half of the frame failures are engineering mistakes, and the other half are fabrication errors. If you try reinforcing and gusseting the area, you are probably more likely to introduce fabrication errors (heat, contamination, etc…)
I also recommend using a pre-butted seat tube instead of the topper. IMO seat toppers are an outdated process: 35MM Ø SEAT TUBE DEDICCIAI BUTTED FOR 31.6 SEAT POST 1.65/.9 X 600MM — BICYCLE FABRICATION SUPPLY
The first time is never perfect, I have to constantly remind myself of that. You already know what to do next time (place the stress on the thick butts), so if it breaks, just build it better!