Good morning,
I bought a used carbon fibre Di2 bike just over a couple of years ago for pretty much the same reasons as you seem to be describing, interest rather than need.
Once the novelty wore off it sat in the house and I use my old steel bike nearly all the time.
As there are plenty of used CF/Di2 bikes on the market which have had most of their deprecation factored in, you could buy one and think
I like CF/Di2 sell it losing relatively little and then buy a new bike, as and when new bikes are actually available to buy. Or think okay I understand the improvements but they are not important or relevant to me.
For me, a hobby rider who only occasionally rides fast and when I do I choose the CF/Di2, the improvements really aren't that great most of time. Added to which I am going from downtube shifters on the steel bike to Di2, so some of the improvements may be there with mechanical STI.
Personally I wouldn't buy a new CF/Di2 bike mail order as I would value the warranty and ease of going back to the shop if there are any issues over the savings.
If you are going to have Di2 then you might want to do a bit of reprogramming, or have help if you really mess up the reprogramming, no chance of that with a mail order bike. The used one that I bought was from the LBS and came with a 3 month warranty which was long enough to cover any faults at the time of sale which was all I could expect on a used bike.
As for Ribble they seem to be moving more and more away from customisation, but this could just be a lack of parts, the days of putting Dura-Ace on the budget blue aluminium frame seem to have gone.
When I ride the two bikes the step down from CF/Di2 to steel/dt is much smaller than the step up from steel/dt to CF/Di2 if this makes any sense. Going up, I feel lots of nice new things, going back down I don't miss them. Although I get why someone who considers "CF/Di2 as normal/modern" would see the step down to steel/dt from CF/Di2 as much greater.
It would seem likely to me that the step up from Alu/mech STI to CF/Di2 would be even smaller than the step up from steel.
Unless of course you plan to go the whole hog and get a sub UCI weight limit bike with ultra aero wheels and only ride it when it's not too breezy .

I have slowly come to the conclusion that road bike development stopped at 9 speed aluminium if you are riding at less than 25mph.
Bye
Ian