Beware "the thinnest possible". It can kill the sound.
A long time ago when I started my equipment installation business one of my first customers had a similar requirement.
I had done most of the house during an extensive refit. A lounge, office, kitchen, bedroom + en suite and a games room all had either AV or pure Hi-Fi installed along with several Sonos music players. Then we came to the dining room. Other than the kids bedrooms, this was about the only room left where tge existing decoration was to remain intact. No chance to channel tge walls then.
He insisted I use a really thin cable. It was about the gauge that comes free with those old ministers stereos. At a guess I'd say it was less than 0.6mm cross sectional area. There was a nice Onkyo CD receiver paired with a couple of original KEF Eggs. Not my favourite speakers but relatively small and discrete, and fine when partnered with a small sub.
All the gear had been tested beforehand, so I knew nothing was faulty. The Onkyo sounded really nice into the 6.5" kitchen ceiling speakers. (Better than the KEFs IMO.)
Come the day of that part of the installation, the KEFs sounded crap. I even took them down from their perches high in the room corners and tried then direct with short leads from the Onkyo. Nope, no issues. Put them back and they sounded crap again. The thin cable was strangling the power. Eggs need a bit of juice to wake up, but these thin speaker cables were the equivalent of a partly blocked pipe.
Have a look at 1mm cross sectional area (CSA) flat speaker cable. The first thing I found via Google was from a company called mkshop. However, theirs is aluminium with an anodised copper layer. It's known as CCA. That stuff is crap. Get 100% copper.
At 20m runs, an all copper 1.0mm CSA cable will lose around 15% of the amp power. Going to 1.5mm CSA won't make a huge difference. Its about a 12% loss.
The numbers will change is your cable runs are longer though.
