Nikon Z7, 7Artisans 75mm f1.25 @f1.4
If you're going all-in on Full Frame, finding an Every Day Carry camera that isn't fixed lens and matches your system becomes a real issue.
Out of the 4 systems, Nikon struggles the most here, lacking entirely in an actually compact body. The Zf is actually larger than the Z5/6/7 series in 2 dimensions, and heavier too. It does pack down smallish with no lens or a particularly compact pancake due to the lack of grip though. Otherwise for current new bodies, the Z5 is the smallest (the original Z6 and Z7 are the same exact size as the Z5). But none of these are small and Nikon has exactly 4 compact lenses, the 24-50 and the 26, 28 and 40's. That does not make for a really flexible system and the rest of the glass tends huge (although the Tamron rebadge trio is pretty compact for f2.8 zooms). The most viable option really is going to be pairing the Z5 or Zf with the 26, 40 & 7Artisans 75mm f2 (which is quite compact) and maybe adding the 7Artisans 18/5.6 fixed aperture pancake as a decent cheap wide. That's a nice small & inexpensive lens set on a kinda chonky body.
Canon's Rp and R8 are quite compact and Canon offers 4 truly compact lenses in the 24-50, 16/2.8, 28/2.8 and 50/1.8. Note that's a range which makes buying all 3 primes more viable since they actually gap well, unlike Nikon which clustered up the two wider ones since apparently Nikon is allergic to wide primes these days. Downside is mostly lack of IBIS, but Canon's 24/1.8, 35/1.8 and 85/2 primes are all 1:2 macro and IS, which makes for another good trio of smaller primes. Canon also offers a fairly small zoom trio in the 15-30, 240195STM and 100-400, all IS, compact, 67mm filter and inexpensive. The 24-105 is mediocre, but the other two are decent (15-30) to excellent (100-400) optically. No IS here actually helps for adapting, since it eliminates the worst part of the experience for Canon users (setting IS focal length). The viewfinder is a very consumer 2.36MP unit at 0.7x magnification. Pricing is very reasonable here, with the older and more limited Rp being the cheapest FF body most of the time (unless the Z5 is on sale) and the R8 also coming in at aggressive pricing. The Rp is also available new for some reason, but be aware it's a very limited body in capability unlike the R8 which is straight up an R6II electronically, in an RP body. Canon has essentially no 3rd party AF lens support though, so if Canon's offerings don't meet your needs, you'll be adapting or looking elsewhere. The R8 however is probably the least compromised as a do-everything body in this class due to the minimal handling compromises, it feels like a small SLR body.
Sony offers a lot of options in this space. On the used market the original A7 and A7R are tiny, but have a lot of compromises from being 10+ year old designs. However the A7R can be quite workable if you are using manual lenses only and can live with the minimal battery life. Don't expect usable AF from the A7R, it's CDAF-only. The A7 has pretty bad AF, but it's mostly usable unlike the A7R's AF.
Sony of course also offers their current A7C series. There's 3 bodies here, the OG A7C, which is fundamentally a A7III stuffed into an A6600 body. No front dial and the worst viewfinder ever in a FF body (2.36MP panel at a ludicrously bad 0.59x magnification). These are reasonably cheap these days, coming in at $200CAD more than an R8 or S9 for MSRP. They also offer the A7CII, which is an A7IV in an A6700 body, and features a now competitive for the segment 2.36MP 0.7x EVF and a front dial. But it's barely cheaper than the A7IV, which itself is overpriced for the performance compared to the competition at this pricepoint. Finally Sony has the A7CR, which is an A7CII with A7RV internals. It's easily the most expensive A7C variant, but is the best actual value since it's significantly cheaper than the A7RV. This is the lightweight hiking/landscape body and if that's what you are going for, the only competition is not much cheaper but has way more limitations (Sigma Fp L + EVF).
Sony also has by far the best selection of small glass between Sony's G series compact lenses, the Sigma 'i' series primes and a myriad of offerings from other manufacturers, especially including Samyang.
In L mount there's options from Panasonic & Sigma. These are unique in not offering mechanical shutters on any of them, which can be an issue, but they're also by far the smallest options available.
Sigma offers the new Bf, the original (and discontinued) Fp and the current Fp L. The Bf is your choice if you are looking for looks over performance, it's a very good looking camera, but compromises handling for looks (sharpish corners) and basically has no accessories possible other than lenses and spare batteries (not even cards, it comes with 230GB of internal storage and a USB-C port). The Fp is not great unless you want the smallest possible camera and don't care about AF. It's CDAF only so AF performance is lousy. The FP L adds PDAF and a 61MP sensor, and without EVF is the cheapest high-res body on the market and also the smallest by a long shot. All these are VERY situational cameras, you will love them or despise them. No IBIS either on any of the Sigmas. Note the Bf has no EVF option where the Fp's both have an add-on EVF/
The Panasonic S9 rivals the Canon R8 for being the cheapest option with good performance, is the smallest option with IBIS, but lacks an EVF entirely. It's got all the S5II toys as well, so it's actually a very capable camera if you can live without an EVF and a mechanical shutter. This is by far the best video body of the lot and is marketed as a Creators Camera, selling up the video and the stills+social media workflow using the Lumix Lab app on your cellphone. NB - There's no hotshoe on this camera, only a cold shoe for an optical viewfinder or video accessory.
L mount is second in terms of lens offerings, with Sigma's i series flashes and a decent selection of other lenses. Note Panasonic's 1.8 prime series (which includes the 100/2.8 Macro) are all identical in size and very close in weight, these are mid-size for the 24-50mm range, but the 18, 85 & 100 macro are compact for the specs, especially the 100 Macro which is by far the smallest such AF macro ever made (and it's good optically too). The 18-40 is also unique, a pancake type UWA zoom.