If you were to build a commuter bike what would it be?

For flat bars vs drop - I've had one or two commutes where I preferred flat bars due to a bit more upright position in traffic.

For me I'd probably consider three choices -

A slightly older 11-speed road bike with mechanical rim braked 105 as long as it has space for 28mm tires

A gravel frame with 35mm+ slick tires, maybe steel

Something like a Canyon Commuter - disc brakes, internal gear hub. Basically a hybrid but no more than 9kg

In all cases I would want full mudguards and a rack

Belt drive and hub gear. Cleaning and maintaining a commuter gets old real fast. As much internal cabling as possible for the same reason. Which is why a cx works as a commuter. Rack full & mudguards. Make sure there's good mountings for lights front and rear that aren't blocked by panniers etc.

Toolkit for punctures and practice only using the tools on the bike.

I had puncture proof tires with some tread. But I used to find the tread picked up glass and sharp stones which then would get punched through the tire if you hit that spot with a pot hole or such. So I'd get slicks. Also fatter for some comfort.

I'd definitely go drop bar for that distance.

My current commuter is a Kona Rove AL, which was my Gravel bike originally. I replaced it for Gravel with a AL Topstone, which also has rack mounts (and is a much nicer, and lighter, bike). I've been using it off road with the rack on, after using it on holiday on gravel roads of France, and it's coped with Irish fire roads.

I personally find a rack far better than a backpack. The other thing I really wouldn't be without now is a dynamo hub set up, with decent light. My B&M one is great even on unlit roads.

I have a Condor Fratello and it's perfect for commuting (even though I haven't commuted in 2.5 years).

28mm tyres, mudguards, rack, but not a tank in terms of weight. Can definitely get up to speed on it. Not necessarily a budget friendly option, but I picked up second had rim brake frameset few years ago and delighted with it

If you are using panniers then I would say that a kickstand is - if not essential- very useful. It's much easier to put stuff into and take it out of panniers if the bike is standing up, rather than having to lean it against a wall. If there isn't a chainstay bridge for a kickstand you can get one that mounts to the chainstay and seatstay on one side.

Yea the Promax callipers aren't great but TRP Spyres are meant to be a big upgrade.

Trying to figure out mudguards at the minute. Do you have full ones on yours? Clearance looks very similar. I do want a rack so the integrated one the Wingees looks good. It's not that much more than buying a pair of SKS chromoplastics and a rack and should be a nice bit lighter. Leaning towards the ones direct from Herkelmann, the dynamo options do make for handy cable routing though it does mean an extra wiring connection around bottom bracket if I go with a mudguard mounted rear light.

I'd say clearance will be fine it can take 42mm tyres so should be fine with 40mm guards and 32mm tyres. I'd say even the 37mm that are on it would fit at a squeeze. I've printed of their template and I'll check later.

Drop bars are the better option if you encounter wind. They also default to being a little narrower, which is good for occasional gaps. Something like a Giant Defy disk would be a good option, relaxed stance and it will take bigger tyres 32s, but defaults to 25s, which I find much nicer than 23s.

I am running 40' Schwalbe plus tour tyres. I think the bike came with Riddler 37' tyres. Using BBB mudguards sourced from LBS which are OK but very limited clearance with the 40' tyres.