In uncompressed mode, the line-input is sampled at CD quality, causing no loss of fidelity.  You are overthinking this.  ADC/DAC conversion is far past the point where you need to worry about loss of quality.  Back in the early days of CD this was a problem, mostly due to analog audio engineers’ unfamiliarity with digital mastering.  But today, the chips are a commodity, and the wrinkles in the process have been ironed out long ago.  

ETA: Funny story.  A person once posted in here how much better vinyl from their turntable into Sonos sounded than that horrible digital sound.  It was warmer, more spacious, etc.  They didn’t know that the analog Line-In is immediately converted to digital, and their ears certainly couldn’t tell the difference.

What would an RCA connection put out to the Era other than "CD quality sound"? Yes, the digital information contained on the CD will be converted to analogue, but will that make it anything else than CD quality sound?

The Era will digitize the sound again for your Sonos system. This will be a lossless conversion, so again preserving CD quality sound.

I'm using a Marantz CD-player (probably twenty years old) to feed my Sonos Connect. You could go the second hand route too - CD players are not the commodity they once where, so there are no really cheap players anymore.

Why not rip your CD collection to a lossless format and add it to a file server?

100% quality and no need to be shuffling CDs and cases to listen to your music?

If you really want to play the CDs maybe try using your video disc player instead of buying new?

All good points to which I will add: Or, just get audibly the same sound quality from the Era from Spotify and the like. Unless your CDs are such that are not listed on any streaming service or your internet service is not reliable.

Ripping CD’s is not the onerous project that it once was. There are programs that have access to large databases of “Metadata” (disc title, artist, song title, etc.) The ripping program will automatically fetch the data. There will be a learning curve that first evening, but you’ll soon develop a rhythm and won’t need to pay much attention to the process. When I rip, I simply stack the CD’s next to my computer and set the CD tray to open as the rip is finished. When the tray opens, I drop in the next disc. It hardy breaks the flow of whatever else I’m working on.

If friends drop by with a few CD’s, I can understand that you might not want to take time out to rip. That’s not my situation.

If you rip the CD’s into a lossless format, the music will remain in the digital domain until it exits the speakers.

Don’t listen to the snake oil salesmen.