Ask HN: Software recommendation to keep multiple calendars in sync as freelancer

Any suggestions/software to keep multiple calendars from multiple clients in sync? Currently i've three clients, two with Office365, one with Gsuite plus my private one. I find myself constantly going through three web views, making or accepting one appointment and creating a blocker in the other two. Running on Linux, a big plus would be that the credentials won't leave my system. I won't mind using e.g. a private google account as "master" to merge and distribute the calendars as long as the others would only see "blocker" and not the details of the actual appointment.