Quite a few people run systems with a Raspberry Pi, a high-quality I2S DAC+amp, and a set of (normal, non-internet-connected, firmware-free) speakers. There's a tiny industry of producing hardware and software for this situation.

"Usability" means very different things to different people (after all, the reason Sonos asks for location access is to improve usability), but maybe look at https://volumio.org/, http://www.runeaudio.com/, http://moodeaudio.org/.

Or just install MPD and Pianobar for that old school experience.

What were you looking to get out of a Sonos? If you just want something that can play music wirelessly from various devices, I'd recommend getting a pair of bookshelf speakers and plugging them into one of (Airport Express, Chromecast Audio, Raspberry Pi[1]). You won't be locked into Sonos and generally bookshelf speakers sound better than all-in-one units. I'd love to recommend some speakers if you're interested (e.g. [2]).

I have run a few Airport Expresses for the past 10 years and love them. Older models are cheap (<$30) and still work great.

[1] https://support.hifiberry.com/hc/en-us/articles/205699981-Ho...

[2] https://www.amazon.com/Micca-PB42X-Powered-Bookshelf-Speaker...

Low cost SBC (Raspberry Pi, etc) and whatever speakers you'd like. Airsonic or a basic UPnP server as your driver.

Take it a step further (if you prefer portability of sound quality) and get an SBC with a bluetooth adapter, and several bluetooth modules which you can rig up to each speaker.

Well, there's snapcast that allows you to relatively simply build an alternative, but I don't know if anyone builds and sells ready-made products.

You can probably go by using a couple raspberry pis (or any ARM board with a jack output), a preconfigured SD card image, and some external speakers. I don't know the price of Sonos speakers, but I don't think it would be much higher. I guess someone could even create a business out of this, especially if tailoring the PCBs to the task.

You don’t mention which ecosystem you live in. I’m very happy with my Apple Homepods. The sound quality is great and I trust Apple with privacy. I don’t use Siri at all (I have disabled the microphone) and so for me they’re purely network speakers which are very easy to use and look after themselves with updates.

I've used Libratone [0] for years. Superb products. Excellent audio.

[0] https://www.libratone.com/

Do you want to stream audio from online streaming services, or just play local files from your NAS/PC?

Bose has a competing range called Soundtouch, no idea what their privacy is like but usability is good.