In general (how many times have I typed this?) dropouts on a line in is an indication of an amount of wifi interference . The amount of bandwidth required by a line in signal is much more than a normal stream, and consequently is more sensitive. You’ve already done some of the recommended steps by increasing buffering and compressed audio, but I’d highly recommend reading the linked FAQ, and applying as many of those potential issues as possible.
At the end of the day, I would recommend that you submit a system diagnostic within 10 minutes of experiencing a dropout and call Sonos Support to discuss it. Although they’ll likely just be able to say the same thing with more certainty than I can.
It’s always possible that there may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution, if it isn’t simple WiFi interference
When you speak directly to the phone folks, there are more options available beyond just the diagnostic analysis.