Following the suggestion by u/Mechanize I am slightly changing the release cycle to a more structured approach:

• I’ll stop distributing app bundles through the open testing channel of the Play Store, because it’s time consuming and pretty useless (it implies that I have to undergo the approval process twice for every build: first when I upload it on the test channel then when I promote it to production);
• I’ll continue using GitHub for development builds, no more frequently than once per week (ideally every other week), typically on Thursdays.
• I’ll be releasing a stable version on the production channel of the Play store no more frequently than once per month, typically the last Thursday of each month.
• in case of severe bugs, hotfixes will target the last stable release, changes will also be also reflected in the first development build of that week’s Thursday.

I’ll stick with semver for version naming, except that pre-releases will have the -pre suffix.