Other than Mirrorless, you described a Nikon D810 w/ the new 105mm f/1.4 or even the older DC lenses.
Sony cameras are wonderful and mirrorless has some advantages for non-action subjects, but I find compactness is not one of them. Everyone has their own personal preferences, but while I can agree that an 1DX or F5 might be too big for some people, I find anything smaller than my D500 or maybe a D7200 almost unusable. And I have small hands.
D810 would stretch my budget, especially with that lens. But IQ is quite a bit better than anything on my shortlist, wow! Going down to more affordable D750 the IQ edge disappears, though, as it is on par with the others on the shortlist as far as I'm concerned.
Aside: there's a Canon 5D Mk IV coming out? At 3,700 EUR for body only
Last time I looked at camera bodies was in the Mk II days, costing €2,500 at launch. Glad I got out of that race. And yet, here I am again...
For me camera's usability is more about the control layout than too small/big size. I shot Canons for years before moving to MFT.
I haven't done bulk and weight comparisons recently, but you're right in that mirrorless doesn't offer much - if any - improvements on size when comparing the same crop factor cameras.
Here a size comparison between Fujifilm X-T2 with 90mm (135mm equivalent), Sony A7II with 90mm (longest Camera Size has), and Nikon D810 with 105mm. Fuji is also lightest at 1,047g, Sony 1,201g, Nikon 1,770g. Pretty striking differences between APS-C and FF. The apertures between the three combos are roughly similar, but the weight and size differential between the Sony and Nikon would be smaller with a native or adapted 105mm lens.
Looking at the comparison I wouldn't want to do travel photography with either of the FF behemoths, so that would limit their use to the studio, keeping my MFT gear for travel and backup. That might be ok, though.
If you decide to go the full frame route with adapted lenses, another lens I would recommend is the Voigtlander 125mm f2.5 APO. It is a great macro but also works beautifully as a portrait lens and short tele. The images have a rightness to them that I have only seen in the Otus and super tele, as well as some MF glass.
I replaced my Zeiss 100mm f2.0 and 135mm f2.0 APO and don't regret it.
Good to see you here, Bernard, still active!
That Voigtlander does look good, and second hand is within reach financially. Seems like it's discontinued, and unfortunately it hasn't appeared once on eBay in the EU in the last months, so would be a long shot to acquire.
In my book the FOV of the 90 F2 is shallow enough. If you want more and you don't mind manual focus (with focus peaking or split view) then you have some interesting options using the Metabones Speedbooster adapter; I use it with a Samyang 135 F2 but to keep it under 135 mm equiv you could use something like Nikon 105 (maybe the DC version) or the above mentioned Voigtlander 125.
Keep in mind only the newer Fujis have double SD card, the X-Pro2 and the X-T2.
Yes, I'd expect f2 (I believe f/3 full frame equivalent) to be sufficient as well. I'm not entirely convinced I'd really need paper-thin DOF, and I have MF film gear for the rare occasion.
Nevertheless, Speed Booster sounds like a workable compromise, but I'd have to rent the Fuji to be able to tell if I can easily and accurately pull focus manually with whatever aids they have. I'm really disappointed with the fiddliness of Olympus OM-D EM-5's aid, which is very difficult to use handheld.
I'm attracted to the X-T2 for the control layout, dual card slots, solid native prime lens selection, and reasonable price. I'll have to confirm how well the control layout works for me by renting, and see if it's any less fiddly to use than my Olympus.
In any case, I think there are rentals ahead, as there are pros and cons to every option!