Where is the significant point of diminishing returns on hi-end turntable?

Get a used (but with a really good bearing) VPI Mk IV table, get what ever arm and cartridge you want (within reason), and viola:

set up your stereo on a concrete floor, put the table into a closet - the isolation you'll get will propel the tables performance upwards several rungs - maybe over $15k worth of table .  Also throw in some small area rugs, and ASC full traps in the corners, 1/2 traps on the back wall.  Lose the turntable dust cover also, just use a sheet of plastic, nobody is going to see it in the closet, make sure the arm comes off the table at the end of the vinyl to save your nerves and cartridge.

My rig is that table, a Lou Souther linear arm, and a Shelter 501 Mk 3, and a second "bass" table MF 7.1 with an AR 33PTG/II.  Linears don't do bass that well, and linears that are pushed by air?  No thanks.

You might want to try for a lower end base table, and borrow/beg other tables to check out.  If you can't tell the difference it's a vanity purchase.  From the 70's onward the best cartridge you can get on an arm that resonates at the right frequency, with the proper alignment, on a belt drive table that has the motor isolated from the table is most of the way.  Then you've got to get a killer step up - most pre-amps can't do the job.  Big transformer fan, but top notch SS units like Pass's do very well too.

I'd ignore MM cartridges and stick with low/mid compliance MC's, and look at the linear trackers, no way an arm on a pivot can touch a linear for quiet grooves and low wear.  My UHQR's sure appreciate it.

Given how difficult it is to make turntable/arm/cartridge/phono stage comparisons in a meaningful way--a point I touched on earlier--CT0517's advice above is pretty good- get with some people--civilians, not dealers-- in your area-West Coast?- who have some top tables and spend some time hanging with them. Most audiophiles I know are very generous that way. You can thus avoid the dealer bias -- I'm not down on dealers, generally, but their job is to sell you what they offer-; you also want long time user experience, which can be invaluable. 
I "get" where you are coming from in terms of not wanting something fiddly but still being able to do routine maintenance and adjustments. 
If you are looking for something to buy and keep, and not be worried about the 'next best thing'  I'd do exactly what you are doing: researching, and spending some hands-on time with some of these turntables. 
I have my favorites and preferences but see no need to name check brands here- there are a lot of good tables and arms out there. 
good luck,
(I was a car guy too, and have fond memories of hurtling around the track in a CGT with Hurley Haywood as my coach for a day). 
bill hart

I can mainly speak from what I own and love: VPI Avenger turntable, Zesto Audio Andros 1.2 phono stage, and the
cartridge options are many: Ortofon, Transfiguration, Miayaja, Koetsu, etc.. all are excellent in their own right but vary based on your personal taste.  The VPI Avenger can be manufactured in belt drive with an analog speed controller by VPI (extra $1000) and gets a lot closer to the magnetic and direct drive, which gets at your diminishing returns question.  The avenger table also allows multiple arms if you own a lot of mono and stereo records.  I would go with the VPI Avenger (future proof), VPI ADS speed controller, Zesto phono stage (tube phono stage with multiple inpits and gain/loading flexibility), and one on the high end carts I mentioned above.  The dealer I work with sells all of it, and definitely think it will be under $25k and well in your budget.  Let me know if want to talk with him.  Best of luck

While it's tempting to focus on money (this entire thread does exactly that), as usual with audio gear and money, other things matter so much more. For example, I'm stunned to find almost no mention of MUSIC in this thread--certainly little if any from the fellow that started it.

And that's where I would want to begin this conversation. I couldn't start to understand his wants and needs without understanding what he started listening to in the first place--why he amassed all this pricey equipment at all. It totally matters whether he loves music (if so, what kinds?); vs just uses it for background, or for system demos (hey, it happens, just like w/Porsches--you think they're all used on racetracks?). Does he play an instrument? What's his connection to music in general?

Even for someone with far deeper pockets than most of  us, simply chasing ever more expensive gear is just an open-ended form of frustration...a gerbil wheel you never get off...unless the "pleasure principal" that underlying the whole enterprise is defined & confronted. Unless, that is, the only pleasure in sight is acquisition of hardware. In which case we're all just flailing in unison on this string.

I have an analogue front end that would be about $25K new - but I paid maybe half by buying used carefully over a few years.

You might want to consider doing that.