By any chance, do you have a more precise ETA for the open source plan by now?
On our project (not the same as OP but still an open-source project), having analysis of PRs only targeting our main branch might be an issue, seeing our CI.
The new SonarQube Cloud open-source plan is launching at the end of this month and will be exclusively available for public repositories. All legacy free plan organizations will be automatically transitioned to this new plan without any changes to their current features. Both custom quality gates/profiles and feature branch analysis will be available.