You've already got the Sonos 1 so you know what that sounds like. If you haven't yet tried it then give Trueplay a whirl to hear the difference it makes. The Sonos 1 or Play 1 is possibly the best sounding smart speaker for it's size and price, but bigger and more expensive speakers do move the goal posts.
The main limitation of the Sonos 1 is bass. As clever as the tech is inside, it can't make up for the small drivers in use. Something like the Amazon Echo Studio is around the same price but physically much bigger, so it's easy to put in a larger bass driver. This is where the sub of the CA/BK combo wins. You can match that by adding the Sonos sub, but it's the best part of £700 alone. Since you already own a Play 1 then the cost to complete the system this way would be £900 (sub + 1x Play 1). If you had to buy this from scratch though the cost would be nearer £1,100. This does keep you within the Sonos ecosystem though.
The CA Min12s (with brackets) and a BK sub would, IMO, fill the space better - particularly at lower volumes where the benefit of 4 sats means there's more points of origin for sound - but a lot rests on the electronic to drive the speakers. You've also then got considerations about the source for the amp and how to control everything whilst keeping it neat.
A pair of Play 5s at a cost of roughlt £1,000 would give you something with the bass weight of conventional loudspeakers with 5" bass drivers, so more in-line with conventional Hi-Fi and less need for a sub, but that doesn't fit with your requirement for small speakers.
To me, the best combination of discreteness and fidelity would be in-ceiling speakers and a Sonos Amp.
Depending on budget, you might be able to do this either in one hit or split it so that you add some extra speakers later when finds allow.
The Sonos Amp is £600, but in one hit it gives you a ballsey power unit and solves source streaming, control and connectivity. Add to this one pair or two pairs of Bluecube BCP65 speakers. These have 6.5" bass drivers, so they do decent bass compared to other in-ceilings upto £350/pr. The BCP65s are £199/pr.
What's more, they're fairly unique at this price level in having 4/8 Ohm impedance switching. What that means is that to run four speakers (2x stereo pairs) doesn't require an amp with a speaker A+B facility. You can take the left channel speaker wire and daisy-chain two speakers from it, each set at 4 Ohms, with the result that it looks like a conventional 8 Ohm load to the amp. Do the same for the right channel with two more speakers and you've got 4 speakers in total running off a 2 channel amp.
In-ceiling speakers do require fire hoods if there's any living space above the area they're installed. That will need factoring in to the budget. Hoody is a brand I would recommend. Hoody 1 is the right size for the BCP65 speakers and it includes some acoustic damping too to help reduce sound leakage up in to any room above. Hoody 1 is around £55~£60 per unit.
Where you're at first fix, you could put in speaker cabling wired for 4 speakers but only install the first pair. Switch these to 8 Ohms at this stage. When you're ready to add the second set then do the cut-outs at the appropriate places with the cable already in place, but then pull down the first set and change the Ohms rating switch to match.
A 4-speaker system would come in at a shade over £1000 + the cost of speaker cable. This would remove the need for a sub taking up floor space. The speakers will go party loud, but they'll also do Hi-Fi performance. Better still, they're very discrete and the headend gear takes up minimal room. You also get to repurpose your existing Play 1 somewhere else in the house.
