In Feb 2021 Google will remove the “allow less secure app access” setting. This will prevent msmtp from sending through smtp.gmail.com.
msmtp supports Google’s preferred OAUTHBEARER method since version 1.8.4.
Could you bump msmtp for Turris Omnia from 1.6.2-2 to a version that supports auth oauthbearer?
https://marlam.de/msmtp/news/
https://openwrt.org/packages/pkgdata/msmtp