Thx a lot for the nice comments and suggestions. Been browsing a bit through my vacation pictures of the last few years, and also been researching some lens reviews for both systems. After a lot of debating, it's pretty clear that I definitely lack an UWA. Tele is something I don't seem to use that often. Once had a 5DIII+70.200/2.8 and got very nice pics at 200mm with that, but I remember not using it all that much.
And while the Fuji is incredible both for colour and handling, I think since this Japan is something that I've had high on my bucket list, I should probably just go for the "best" image quality. Colors I can fix in post should Sony disappoint a bit (it usually does not, I've set my "standard" creative style to -2 saturation and I like the look of that, 0 is too warm for my taste). The Sony with the Tamron isn't that much heavier than an XT3 with 16-55 and has IBIS + longer battery life which is pretty important too.
So I'm quite set at the following setup :
* A7III (own)
* Tamron 28-75/2.8 (own)
* Tamron 17-28/2.8 (buy > gives me UWA + can be used indoors as a "semi" prime)
* Zeiss 55/1.8 (own) > if I'd want some portrait/light tele stuff indoors, but probably won't be used much
I think this should be enough while keeping stuff somewhat light enough. I'm pretty sure the 24/GM will once be in my bag, but the above will "only" cost me 1.000 euro. For the Fuji I would need the 10-24/4 to get the same, which costs about the same too but then I'd be "stuck" at F4 on APS-C which is a bit limiting compared to the F2.8 of the Tamron. I never owned an UWA lens, ever... (been photographing over 20yrs) so might be a nice way to step on new ground too.
I'll definitely keep using the XT3 for family stuff when I really don't want to carry weight, I'm even considering getting the 23/2 for my 'daily/small/street' stuff. But I think it would be kinda silly to leave a great FF sensor at home for a few grams difference, especially for such a special trip.
Also very interested in the XT4/A7IV... very curious what that would bring. I really really hope Sony brings film simulations and in-camera raw processing, with a better EVF/LCD. That would be awesome.