It seems that I can set it from the Android app, not from the web interface.

Does anybody know what the (potential) repercussions are of enabling PQC? I would hate it if I lost access to some emails this way.

edit: see this message. The feature is temporarily disabled as some users of Proton Drive for Windows (Tux is still crying..) reported issues with sync after enabling PQC. PQC only applies to new encrypted emails going forward.