Sorry for this long essay, largely thinking aloud, I am trying to ensure I respond to all the various comments and suggestions people have helpfully made....
I think I am beginning too get better understanding, and nearing a decision. If there's anything anybody thinks I have got anything wrong please speak out!
First, I don't think there is going to be any way with a compact camera to achieve the focus (&defocus) depth of field control that I remember from that old Zorki 4 of my youth, reason being the small sensors. So far as I can ascertain, in relation to Depth of field, an f2.8 lens on a compact is akin to f16 on a 35mm, whereas my old Zorki went all the way out to f2.
DSLR cameras tend to have much bigger sensors, similar in size to 35mm negative. Thus they are capable of similar DoF effects to a 35mm camera. But I really want the long zoom lenses that come with compacts which are not that practical with full size sensors as they would be massive, and would probably cost more than my entire pension pot.
While the suggestion of rangefinder-style Panasonic GX 80 did catch my attention, I think the same logic applies - sensor too big to work with (cheap) long zoom lenses.
So I am reluctantly prioritising (cheap) zooms over sensor size. Nevertheless, whilst accepting that depth of field effects effects will be very limited with a small sensor, they'll be even more limited with a small sensor and
a small aperture. For that reason, I'm now gravitating strongly towards Bridges, as they tend to have larger lenses. Strongly considering either the four year old Panasonic FZ200 or maybe FZ330 which is a bit more recent. Both benefit from an f2.8 lens that stays at f2.8 all the way to full zoom.
I am further reasoning that, even apart from my obsession with Depth of Field, the benefit of f2.8 at long zooms should be that more light gets in, which should allow a faster shutter speed, hence more forgiving of camera shake?
For a tempting 20% or so on top of price over the FZ330, I can get to the FZ1000. FZ1000 has a mere 16x zoom vs 24x, but I could live with that. More impressive, in relation to my DoF obsession, it has a 1 inch sensor, more than twice the size, which ought to be a big improvement. But it still falls short of a full size sensor, still no match for that old 1970s Zorki, and is getting on for 3 times my original budget. So I think I may show some restraint, and stick with the 330 as best compromise.
Regarding the extra bulk of bridge (or even SLR) vs compact, I'm reasoning that (1) I still have my old compact, eight year old Panasonic TZ5 and (2) mobile phone cameras have reached the point of being almost adequate sometimes and continue to improve. So even though there'll times when I can't be bothered to take the bigger camera with me, I'd still have a compact alternative.
Regarding viewfinders, despite my scepticism, all cameras on my shortlist have decent (I think) viewfinders, just in case I change my mind on that score.
Regarding WiFi and Focus-Assistant App... Panasonic cameras that have WiFi can be remotely controlled by a smartphone App, which may or may not be useful, I have yet to assess. I jumped to the conclusion that Focus Assistant App used the same technology but of course that is not the case, I think it is a stand-alone App. It looks good but does not seem to be available for iOS, which is disappointing for me, as all my existing phones & tablets are Apple.
Thanks again for all inspirational input.