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The Tarmac SL6 with 105 hydro is a really good bike, you can find the 2019 discounted in many places. It's light if you compare it with similar bikes from other brands, but going from rims to discs will add about a kilo.
The BB is not threaded, only decent ones I can think of is Bowman palace disc and Trek emonda ALR, but these are alloy frames.
Netakure-
Ah that's what I thought yes. Seems like the best option right now. I had a look at the groupset weights and discs add very little. For 105, apparently only 25g more. A little extra wheel weight too, for example disc Zonda is 135g heavier.
https://ccache.cc/blogs/newsroom/2019-road-groupset-weight-comparison
I'm sure with this particular bike it's the lower grade carbon that adds some weight. And it does have a threaded BB. See photo below and the spec sheet and reviews.
Super_653
Something for fast and short training rides, and maybe some longer ones mixed in too. I loved my previous SuperSix, but BB30 and rim brake.
Looking to future-proof and be able to maintain it myself as much as possible, so that means 12mm axels and discs, and threaded BB. Have been drawn to the Specialized models because they have that one which ticks those boxes, and the models above (too pricey) come with pre-installed Praxis Shimano adaptors.
I can gradually upgrade parts, and if I ever want to upgrade the frame, I can just move everything over. If I go for rim brakes again, then I have to change the wheels, shifters, and brakes later if I change to discs. Also I like the idea of not wearing down rims, and could even try carbon for the first time as fancy training wheels.
See the attached pic for the kind of thing I would aim for, but just with 105 mechanical to begin with, and maybe Zonda disc wheels. That's a 58 and looks big, coming from the more traditional geo and shorter HT of the SuperSix. I'm 189cm and could go 56 or 58.