I'd never bother with a mech disk, I'd just stay with rim.
Elaborate because my actual experience has been different. I used TRP Spyre calipers with SRAM eTap shifters, Jagwire Elite Link cables, SwissStop Disc E pads. Initial lever throw had slightly more resistance, but once the pads contacted the rotor, the modulation was there. Hy/Rd calipers take things a step farther by making the lever throw feel even more like a fully hydraulic system. The Spyres required manually adjusting pad distance every month or so, but that didn't bother me much. You knew from lever feel when it was time to quarter turn the adjustment screws. Now that my new bike has eTap HRD, the improvements in brake feel are not a revelation. Everything is incrementally better. The biggest change is the initial pull on the lever being less sticky.
I would definitely recommend either TRP Spyres or Hy/Rds to anyone looking to reuse their existing brake/shift levers on a future disc build.
Sorry, I should have made myself more clear. The situation is that I live in a dry area, and I'm more than happy with the performance of my Shimano 105 brakes on DT Swiss rims, in fact I'm thinking of upgrading to a lighter rim brake.
However with the possibility of running a four pot MTB DH caliper on the front of the bike, then I think the 300 grams it would add would be worth it just for maybe a bit of engineering curiosity, at least to investigate if it's really a feasible or a desirable idea. As great as the reputation of TRP Spyres, and I'm sure they are great brakes that would make a fantastic affordable upgrade especially in a hilly and wet region, it's not something that interests me in my current circumstances.