But taking this places requires two bags as I can't fit two weeks+ of clothes etc in the case with it. Hence the thinking about a 20" wheeled bike that might go in my XL North Face Duffel bag with enough civvies to do a trip and not have to pay for a second checked bag (which invites questions / complaints from work). Drop bar Di2 Ibis gravel bike build is just under 9kg / 20lb, which makes a difference when you're trying to get everything under 23kg in one checked bag plus hand luggage, but space is still my enemy.
I can see where you are coming from. However a bike sharing the inside of a soft bag with other stuff is not a well protected bike, so the thing should probably have to be inherently knock resistant.
Arguably if you had a better case/bag for one of the bikes you have at present, this might also solve your problem. Or a bike that takes up less room in the case you have, thus allowing more space for clothes etc...?
Things that take up space inside bike boxes/bags include
- fat tyres
- large diameter wheels
- disc brakes
- wide range derailleur gears in the modern vogue (i.e. with large sprockets rather than small chainrings)
- carbon or aluminium frames (which have fatter members than steel ones)
- dropped bars (esp if fitted with modern STIs, which are very bulky things)
- big flat pedals (which are not QD and/or don't fold)
it all adds up. So something like my clonky old BiFrame or an airnimal will take up less space, but if you want something optimised for compactness, you end up with a Brompton more or less, which has none of the above issues rather than some of them (or most of them as with your current bikes). All bikes are a compromise, so it is just a question of what kinds of compromise you are likely to be least unhappy with.
Reducing the argument to its essence, if any bike is better than no bike, it is simply a question of choosing the best bike that you can reasonably take with you.
Could you have a clothes bag which was permanently attached to the outside of your bike case? Would this solve the luggage issue?
cheers