I think I'm more so trying to talk myself out of doing it.

I can see lots of benefits, but the drawbacks are...
Cost - significant, but would hope to last me 5+ years before I feel the need to even consider looking for a new camera.

Size - the reason I bought the A6000 was because I stopped enjoying taking my K200D out with me as it was too big and bulky. But when I didn't I then regretted not having it. So I went for a mirrorless APS-C camera. Going full frame (even mirrorless) pushes me back towards a larger camera. Though I am much more enjoying photography again these days so I'd hope I'd get past the size.

Yeah, I'd say size is something you get past - though I did get myself a decent compact to always carry around for similar reasons, but having the choice I find I don't use it as much as I might have. Still enough that I'm glad I have it, but without it I'd regret shots that I otherwise decide are not worth taking. I always take the D750 when I know I'm going to be doing photography.

I do think in your position I probably would wait a year or two for the next generation before upgrading, (unless you're specifically hitting the boundaries that the A7iii definitely overcomes).