With your background, I think you are more than capable of learning on your own. $3500 and 1-2 weeks of your time would go a long way if you invested that into yourself.
There are plenty of very friendly frame builders who lurk on the forum and would be happy to answer questions. Including me - almost every week I have 1:1 meetings with builders to exchange information.
Brain dump:
Design:
- Dropouts: PMW rocker FM dropouts to avoid having to use a brake mount fixture
- Design for 1x GRX to give you the most wiggle room
- 700x45
- Chainstays - I would need to what pre-bent would work with rockers, 700x45, 1x, and 430CS
- SS - I would get custom bent straight guage ones from bike fab supply
- TT, DT your choice
- ST - Tange externally butted:
Mitering:
- Paragon tube blocks (to help keep your tubes in phase when mitering)
- Printed templates from BikeCAD or Fusion360
- pipe cutter for rough cuts
- hand files + hacksaw for mitering
Tube Prep:
- deburring tool
- Emory cloth (sandpaper)
- Small sanding drums (to get the mill scale off the inside of the tubes)
- Acetone/IPA
Fixturing:
I think if you set realistic expectations for frame tolerances, have accurate miters, and some critical thinking, you can get away without specialized equipment. see this thread
- 1:1 printout for the front triangle
- “hocky stick” for rear triangle
- Dummy axle
Welding/brazing:
not my area of expertise 