Thanks you so much for your answers. I'm deeply sorry but I'm afraid that I slightly underestimated the load when shopping : it'll be more like 20 or even 25kg max rather than only 15 (I can fill a cart like this one :
https://www.amazon.co.uk/Polironeshop-Geko-Bike-Trailer-red/dp/B01C4CQK5K) plus a backpack. And I'm not sure that a bike like my current one would stand this weight if it was possible to hang it on the frame. My current bike is actually the following, so you can get an idea :
Perfectly horizontal top bar with no slopping, with a front carrier, heavy saddle, lock, Abus secured wheels, with lights and equipment, I'm not sure it is the most light and agile and responsive bike ever. I could actually stand to loose a bit of agility if it was to improve sturdyness. Indeed, I'm 1m80 and 85kg and when I push hard on the pedals, the whole thing twists significantly.
I also used to own a 15kg gear-hubbed (Alfine) Creme Ristretto. It's been very helpfull to carry my daughters as well as groceries, but I didn't enjoy it and sold it (very boring bike IMO, and unbalanced because of the rear hub). Now, I also know that I prefer a derailleur (really a matter of taste, no argue about that) but I have to admit that I really enjoyed the hydrolic disc brakes !
I'm looking for something half way between both, probably a closer to my current yellow and red one, but with every kind of eyelets to be future-proof, larger tires and sturdier, without being too much boring (I think that PH perfectly gets my point). And I'm now open to 26" (that's why this post) while a week ago, I just even wouldn't have through about that : a bike is 700c, isn't it ?
@Anto : my point about larger tires is not about 26" beeing strudier : it is more about pinching-flats occuring frequently if I ride more than 10-15km/h on some portions of my commute, and I want to avoid that without needing to brake and start again every 50-100m.
@Lewisjnoble : 12.5kg sounds ok for me. Did you own bikes which could be compared to the ones pictured above, so as to get a reference point about agility ?
@PH : do you think that a Club Tout is more fun than a Sherpa (both with lightly builds) ? And you are right, I have to think about the bar thing. My point is that I find flat bars uncomfortable (during longer rides, painful hands and forarm exterior) and drop bars are out of the question when in dense traffic (I've been the witness of an accident where in the same situation, another biker couldn't brake in time while it's been OK for me, hand constantly on the brakes). So I'm looking for something comfortable, but accessible brakes. Any idea ? I dislike comfort cushions and like to handle round bars because I feel more "involved" this way.
Important : due to room constraints (collective and small bike room for the whole building), I only keep 1 bike, so it needs to be very versatile, just not too much boring.