What is the most “speaker” you would pair with the Sonos Amp? What’s the logical cut off point?

There is no such point in price terms; any speaker that does not need more power than what the Sonos Amp can deliver will work as well as it is designed to do, regardless of what it is sold for. Note also that just because it is expensive does not make the speaker a better performer than even a much cheaper one.

I know it is hard to pin down, I just imagined there may be a class of speakers where it would be common sense to step up to something else, as powerful as the Amp is. Just for future purposes of course, theoretical. The Amp powers my 685 S2 beautifully.

That is a myth used to part you with more money that you need to spend. The world of high end audio is riddled with such "selling" and the satisfaction received by overspending is also psychological. The state of today's amp tech is such that it is not rocket science to make an amp that is as good as any other of the same power rating as that of the Sonos Amp.
What you might get for the higher spend is a longer service life, measured in decades, with part replacement support. But you could just replace the Sonos Amp every time it fails about once in decade, and still come out ahead on the money front. Also, the Sonos Amp will not give you the satisfaction - again psychological - of heft, brand name, dancing VU meters for each channel and other eye candy.

Also consider this: a Sonos Amp or any other, with passive speakers at the business end, is obsolete technology, essentially. And the one that has superseded it, that of active speakers, is also no longer expensive. Which is why even the play 1, properly set up and stereo paired, sounds as good as it does. And which is why I would expect a 5 pair + Sub to sound as good as your set up, at a lower price.

That makes sense, I know audio can be a pretty deep path with diminishing returns at times.

That’s why I love the Sonos eco system, very good sound that works without having to tinker too much. It’s fun to tinker, but sometimes you go down that rabbit hole and never are satisfied, at least in my case.

You are by no means alone; there are some, like me that went down that rabbit hole and found a way out - it took me a decade to do so and discovering Sonos was a large part of how I managed. Some never come out of it, to the dismay of their spouses.

Also consider this: a Sonos Amp or any other, with passive speakers at the business end, is obsolete technology, essentially.

Amp + passive speakers still has it's place in many situations. You don't want to use active speakers outdoors or in ceilings. I'd also say that it makes sense to use passive speakers for very large indoor spaces, or HT setups where wiring speakers is not an issue.

These sound really great with a 100 Watt amp, 105 dB efficiency: https://www.klipsch.com/products/klipschorn

I had the slightly smaller LaScalla and it would raise dust from the carpet on 90 watts.

I was thinking about these:

https://www.soundandvision.com/content/hart-audio-dampw-aural-pleasure-speakers

In gold, of course.

Lol. The molar on its side description is apt.

As to passive speakers, they are still relevant where it is a challenge to supply power to active units, typically outdoors. But the active tech itself, with active crossovers that can easily take sophisticated DSP on board, and amps dedicated to the power needs of individual drive units, has more to offer a good designer than end of the line passive speaker tech, and now, at low price points as well. Passive tech offers more scope for user choices for amps, but that comes with the associated pitfalls of impedance and signal strength/sensitivity mismatches and that can result in less than optimal results.

As to the OP, the 685 are seriously good speakers, so I don't see any need to ever change except boredom with the somewhat plain looks. I am not familiar with that Sub, so I would make sure that it is adding more to the sound than it is taking away - except in rare cases like a true play tuned Sonos Sub, for music play there is always a possibility of loss of integration in the sound/image. The 685 by themselves do a very good job of low frequencies and may well offer a more integrated/natural stereo image than that with the sub added to the mix.