We know from OP comments that this is for rim brakes - might be good to have that edited into the Q

Not an answer to your question, but: rather than single speed, look at fixed gear. On a single speed you can freewheel; on fixed gear you cannot. You are then getting a workout on downhills, too - and learning good souplesse (to be able to spin legs at 160rpm safely) or backpedalling (to slow yourself down). The benefit of single speed vs derailleur is lower complexity. The benefit of fixed gear is increasing leg strength. A similar effect happens on indoor trainers, where pressure must be maintained on the pedals or the turbo slows quickly.

"The one factor that ultimately matters is the average speed" No. Depending where you put the power (descents/climbs/headwind/tailwind) on all but a flat, windless course you can achieve the same average speed with a wide variety of powers. It's power at the pedals that matters, in terms of a workout

@NathanKnutson In the second image (with the hand with clearly visible fingers) there is what looks like a compression slot in the seat tube - I've not seen these on carbon frames before (but I have a lot more experience with steel!) - I thought that would make it easier to compress the seat tube, which (IIRC) isn't great for carbon - are these common?

At University, a friend’s bike had a crack in the down tube just above the bottom bracket. As it failed, the bike got lower and lower until she couldn’t turn the pedals, over several minutes. (This is an anecdote, not an answer, and yours may of course fail differently)

"You may ask why not put the one brake on the rear" It is also to have two independent braking mechanisms - if the single brake is on the rear, all braking happens through the rear tyre. With the single brake on the front, you can brake through both wheels. This is also (two independent mechanisms for braking) why such bikes are legal (in the UK, where I am, at least - I don't know about other jurisdictions).

"If you choose to ride around without a rear brake, that would be inadvisable and dangerous to you and to others. Not recommended" It's also (effectively) illegal in many countries. (And I wouldn't consider this "not bad news"!)

"who in their right minds would cycle along a dual carriageway with 70 mph the normal speed?" There are several such timetrial courses in the UK! (as to whether the people who do them are in their right minds...) (and yes, several of these are no longer used, including the old H25/14 course on the A34 west of Oxford)

+1 for soldering: nice clean end, stick it in a tub of flux (by moving the tub!) then solder (with leaded solder, but the quantities are tiny)

How flat is your commute? The range of gears you need will depend on how much ascent and how much descent there is, and how steep those are

@MaplePanda the cables are thinner, the connectors are different, but the electrical signals are the same (to the best of my knowledge) hence a passive adapter exists (assuming it's passive given it's similar in cost to the cables themselves!)

The EW-SD300 E-tube wire standard used by the R9200 and R8100 groupsets isn’t new, it came out with the EP800 E-MTB groupset in August 2020. And yes, there is an adapter, the EW-AD305, but (as with many bicycle components at this current time) good luck getting hold of one right now. And yes, Di2 uses an interface based on the automotive CANbus, and has done for years, but given the existence of the adapter, R9100 and R9200 should be electrically compatible. Hopefully people like GPLama will test this when they get their hands on R9200/R8100.