Breaking into the full-frame camera world can seem daunting, with many camera bodies starting deep into the US$2,000 territory. That doesn't mean that there aren't cheap full-frame options around though. Here are three of the best mirrorless cameras that you can buy for under US$1,500, often with a kit lens, to boot. https://www.notebookcheck.net/3-of-the-best-mirrorless-cameras-for-under-US-1-500-with-a-full-frame-sensor.710797.0.html
To anyone reading this, please don't do this to yourself.
A7 II is old MILC with a lot of quirks of early MILCs and generally not really nice camera to use. RP on the other hand has worse dynamic range than APS-C cameras (compare it to let's say Fuji X-T20 and Fuji wins by around half of stop anywhere between base ISO and 1600).
Z5 is only decent camera in the comparision. If you really want to push full frame, used Z6 and maybe A7 III (depending on deal) can be recommended. I would however personally go for APS-C as FF will also have more expensive lenses and if you push budget, FF is not the way to go.
As for APS-C, there are Sony and Fuji with mounts full of own lenses and third parties. Sony has cheaper (and usually bit less premium) first party lenses, while Fuji first party lenses can push into FF territory with price. For budget focus on Sigma, Tamron and Viltrox lenses. Canon and Nikon can't really be recommended for APS-C options, unless you have older adaptable EF/F glass.
Z5 certainly isn't waterproof. No digital camera with an interchangeable lens system is. What you meant is water resistant. It can withstand getting caught in an average rain storm, but probably not being hit with a jet from a hose nozzle, and certainly not submerged
The Canon R8 is much better than the R or RP, with excellent focusing and a very capable sensor. If you're going to recommend a Sony, the A7iii is around $1,700 now. Its predecessor shouldn't even be in the conversation, as another commenter noted.
"...wins by around half of stop anywhere..."
I wonder what reasonable picture ever taken would be ruined or even noticeable with 1/2 a stop less dynamic range, especially when the majority of pictures today are viewed on phones / computers off social media sites?
It's crazy to not include the R8 which is by far the best full frame camera for under $1500. It has crazy specs.
Don't think Z5 is waterproof :-D
Weather sealed is the word you are looking for.
Best entry to FF world for anyone is always mostly through used gear. The likes of mpb.com gives you 6 months warranty and you can get some FF bodies below £200. The beat value seems to lie with Canon EF as there are many many great quality lenses at reasonable price to choose from. I.e. brand new nifty fifty 50mm F1.8 STM goes for about £110 on Amazon. The superB 85mm F1.8 USM for your portrait needs is under £200 used. What you sacrifice at times is 4k video, however you often get great quality 1080p instead. With modern denoising software like DXO's Deep Prime you don't have to worry much about low light performance too, or get used 6D mk I and you don't have to worry at all.
Take care.