Well, in what direction do you want to take the bike? Road speedster, gravel king, touring demon, or city prowler? Have you thought of dispensinng with racks, and using instead bikepacking bags? What's your vision, man?

"...
-new drivetrain. 2x10 105 with Tiagra flatbar shifters
... "

The latest 105, series 5800, is 11-speed. The latest Tiagra, series 4700, is 10-speed, and has a cable pull that is not compatible with previous 10-speed road RDs. Caveat emptor. I would just go with Tiagra 4700 all around. Or, if you don't need a very high top end, a simple 36Tx11-36T MTB drivetrain giving you roughly a very respectable 25-82 gear-inches.

Are you sure you want a tire as narrow as 1.2"? I myself would mount the wider Schwalbe Kojaks or Marathon Supremes. If you want a little more style and cush, I'd mount a rubenesque Maxxis DTH 2.3" fatty with trick tan sidewalls. Like so...