You’ll probably not gain much other than niceness in adding Di2 or another gear but if its speed you are after wheels would be your best bet, but if you are just after a smile on your face I’d put Di2 on (it would put a smile on my face any way ).
My bike is a 20 year old Calfee carbon lugged frame. Currently running Ultegra Mechanical. I did just up grade to Reynolds aero wheels, but can’t justify changing the bike. I agree, the best bang for your buck is wheels.
Where are you riding? I’ve been on carbon wheels and carbon brake tracks with Campagnolo red pads for 5 years now without a single problem. If you get the right pads you’ll have no problems with modern wheels.
I wouldn’t touch the Bullet wheels with only a 20.5mm external diameter. Too narrow IMO.
I have Farsports 25mm external (19mm internal) with 25mm GP5000s. They fit perfectly in my older rim brake frameset.
If that is a concern of yours, why not give the bike to a shop, have it serviced and save the rest of the money for a road bike with disc brakes? Disc brakes work much better in the wet and you rims are no longer a wear item.
Your groupset is fine. I didn’t see any improvement going from 5800/6800 to 7000/8000 series groupsets. Stay on top of maintenance, that’s it.
They aren’t wide enough to be aero with anything besides 25c
Cheap ones are fragile
The heds are decent (I have them) but aren’t competitive against a nice all carbon wheel
Nice all carbon rim brake wheels are still expensive, and narrow, and honestly aren’t going to be ideal on wet gritty rides.
105 is fine and probably not worth upgrading, but it’s heavy.
I agree with the others that it’s time to move on to a modern bike with disc brakes if possible because the world has moved on and you get better braking, with all the benefits of nice carbon rims, that are wide enough to take advantage of modern day 28c tubeless tires, and also being aero optimized for that.
My point is upgrading. At best a nice set of carbon/alloy wheels are a marginal improvement over nice mid (30-35) section full alloy rims. The nicest carbon wheels are a slight upgrade but a downgrade in braking so they’re a wash, and expensive. What’s the point.
These should fit most frames and the braking surface is definitely good enough. They’re also not going to overheat on any descents in England. Use the right pads and you’ll be right as rain, so to speak.
I upgraded my Belgium+ wheels to modern Fulcrum carbon rim brake wheels. I’m also happy with my frameset and group set and my options for nice carbon rim brake wheels are diminishing every year, so I went for it while I can. It really finished the bike and I don’t regret it at all.