I can't say if it's the best, but my wife and I each got this one and have had no problems with it. I'll also recommend a stupidly simply add-on called the Dongle Dangler. I use one to keep the adapter on my keychain so I always have it handy!

This is the one I'm using. With built in DAC chip. No background noise and overall working quite well with my S20.. This is the ugreen usb c to 3.5mm audio adapter (this is the version with DAC and compatible with s20)

Tying a bow around this (for now!), hopefully it'll be of use to someone.

I'm not sure what's different about the headphone jack in the back of the Bike+ screen (and, by extension, the headphone jack at the front of the handlebars, since that just plugs into the back of the screen), but no matter what I tried, I could not get the audio to my Sonos Amp. I'm not new to this game, I've had Sonos since 2005, and have dabbled in whole house AV for years, but this got me beat. I tried multiple cables, TRS - TRRS adaptors, but nothing, just get static. If I plug in a regular set of headphones, no issues at all.

Anyway, I figured it's worth trying the USB-C port on the back, and, sure in enough, it worked instantly! I used this $10 USB-C to 3.5mm converter. If anything, the quality of the audio is better than it was from the original Bike, the stereo separation (overhead speakers) is much better.

The only downside of the USB-C option is that it's 'always on'. The Sonos Amp normally starts up when it detects an audio signal on the line-in, but it's always there via USB-C, so the Amp never goes into standby. So my final config is:

Bike+ USBC -> USBC-3.5 adaptor -> 3.5mm to RCA -> Line in on Sonos Amp.