Road bike purchase advice

Focus Paralane has threaded bb and endurance geometry. Available in your price range 😉

Plan B would be to just get a second set of wheels for current bike. Fit one with race tyres and the other with puncture proof. It's amazing the difference tyres and wheels make.

Sonder Colibri.

https://alpkit.com/collections/sonder-colibri

Don't fixate on weight, just enjoy the ride, which on Ti is sublime. Here's mine.

Take a look at spa cycles - plenty of choice and options.

So many choices! Have a look at Fairlight, and also Mason.  A lot of Mason bikes are over £3k but they make steel and ally ones that you should be able to get for that.

I wouldn't focus on DI2, yes it's nice, but its a compromise that would effect the frame and wheels, which most people say are the most important areas to focus on.

I have a Giant Propel which has been my road bike for 9 years.  I still get a kick out of riding it, but it's not great in winter on the roads we all have to put up with.  Road cc ran a story on using a gravel bike as an all rounder & winter machine.  They picked a Specialized Diverge, and almost on the strength of that story, I started to look for one on Ebay.  I managed to find one for £1200 not too far away from me, and it is a fantastic machine.  It was set up for off roading, so came with 45mm knobbly tyres.  I switched them for 35s, put mudguards on and just wish I had gone this route a lot earlier.  There's a little suspension movement in the headset, 20 mm, and this coupled with wider tubeless tyres, the compliance of the carbon frame, makes it a joy to ride when the conditions are not ideal.  Disc brakes too.

I ride a Cannondale SuperSix Evo, and I can confirm it’s racy but surprisingly comfortable for long rides if you dial in the fit. For your needs, the Cannondale Synapse 2 is a fantastic endurance bike can check here https://ebiikesshops.com/—less aggressive than the Evo and great for rough roads.

A SuperSix is definitely not an all-rounder.

Bikes like the Defy, Synapse and others described as 'endurance' would be a good starting point. Boardman and Cube offer excellent VFM. Ribble have already been mentioned.

Other brands will have similar models. One thought is that for a do-it-all bike you might want to look at something even less racy. The Fairlight Strael has lots of fans, though it'll be more than £3k if you want Di2. And there are the all-road, adventure and gravel models, with bigger clearances which will allow for mudguards and/or wider tyres for greater comfort. Cannondale Topstone, Trek Checkpoint and Sonder, for example.

I wouldn't ignore aluminium / alloy frame bikes such as Giant Contend and Trek Domane AL. You get more bike for the money and would have some left over for upgrading wheels or other items and accessories.

Look in the reviews and buyers guides sections of road.cc for ideas.

My money would go to the Giant Defy range. The Mrs has a 2023 model last year which has 105 di2, capably handled bike-packing LEJOG, and has lots of room for easy modifications (stems, wheels, tyres etc). They seem to have managed the best combination of speed vs comfort to my eyes (and derriere), and if you google some reviews you'll see others agreeing. It really is a fantastic bike, and when it came to getting a new bike for myself (after it was stolen) I've had to spend significantly more on an equivalent brand just to avoid having the same bike - sigh.

A 2023 model will get you well under the 3k mark with a high spec if you're content to have a bit of exposed cabling. It will be carbon, and I note your concerns around longevity, but would counter that its come a long way in the past ten years and personally I feel you shouldnt discount bikes on this basis.

I agree with your assessment on the Supersix - this is a pretty aggressive racing bike and, whilst lovely, I dont think its really what you're looking for from the description. 

I'd recommend test riding a Defy, most LBS's stock them, to dial in the right size for you (Giant's often come up a little larger than other brands in my opinion). I dont think you would be dissapointed.

It's obviously a very personal thing but if I was in the market at your budget I would take a serious look at the Ribble AllRoad SL Pro, currently at £2699 for 105 Di2 groupset with Mavic Cosmic wheels and 32mm tyres. Threaded bottom bracket too!