I'm looking to move on from my current Gitane tandem, basically I've gotten distrustful of a 1" threaded steerer on a bike that weighs a lot with a team that, despite my best efforts, keeps going up in weight. This has me looking at https://www.dolan-bikes.com/tdr-frameset/
I'd considered getting a new fork but estimates seem to be in the 400-500.00 range which just seems like a lot just for the sake of keeping a 40 year old french tandem running. Mostly I'd swap over the newer parts I have; Velocity disc wheelset, truvative elita crankset, microshift 3x10 shifters. This would let me use less flimsy old aluminum bars. and have better stem adjustment than trying to find a french stem of the right dimensions. I'd need to/want to get disc brakes which has me thinking of going to a 2x12 system with hydraulics with the obvious issue of trying to figure out the crankset, main solution I can think of is grind down the spider enough to put 12sp chainrings on it and have proper spacing which wouldn't be too hard to do. Either way, I'd suspect it has to be an improvement over old french steel but also open to other reasonable options.

Reasonable Option #1: Buy a mid-late 90's-early aughts Burley or Cannondale rig. Customize (minimally) to suit. Bob's your Uncle. Reasonable Option #2: see R.O. #1. Reasonable Option #3: see R.O. #1.

What do either of those options gain me? I've had a Burley and really didn't like the BB setup, tried modifying a cannondale mtb tandem but couldn't get good positioning on it being the wrong size, but either I'm looking at 1000.00 or more, c-dale seem to be 1200+ most times. Both are going to be old with unknown levels of use vs new frameset and mostly lighter used parts fitted out with new parts as needed. Can you explain why they would be better?

I see Burley's of the kind I'm thinking of (26" Road) for $500 - $600 all the time. I've not priced road C-Dales but $1200 would be high. $1200 would be worth it for what you'd get. You don't think you are going to put way more than that into your built up Dolan frame?? You certainly will. There's always something you didn't think of. Truvativ Elita is entry level stuff. Not judging, that's what is on our road tandem but it's not worth bringing over to a new frameset. You've been stopping fine for years with v-brakes. You don't need discs, hydraulic or mechanical. If they happen to be OEM on a bike you are interested in, why not, but don't worry about it. I haven't even scratched the surface, and neither have you. You've got Microshift shifters or brifters? What happens if you go to 2x12? And why would you try to make a 3x10 triple into a 2x12? A 20 year old tandem is not so old. They are usually much better taken care of (and ridden less) than a single of the same age. I would at least consider it. How much is that frameset anyway? Looks like a road frame. Are you trying to make an MTB or a Road tandem?