I did a flat to drop conversion back in the days of mechanical discs. Those were the days. Honestly, the posters suggesting a drop bar bike purchase are not wrong. There is, however, another way if the o.p. is willing to 'downgrade' the braking to mechanical disc. They are perfectly fine. Brifters to suit a set of TRP dual puck disc (Spyre) should be reasonable as, of course, are the Spyres themselves. Drivetrain will not need to be touched. The last couple of projects I've done have involved used parts. I never considered used components before Covid. Now I buy all kinds of things used and I'm saving a ton of money.