Syncing Google, Outlook, and iCloud calendars,has anyone found a solution that actual

Hi, I manage several small hosting companies at the same time (three separate entities, separate billing the whole mess) and syncing calendars among team members has become a real headache. We have people on Outlook in accounting, others on Google Calendar for tech support, and one of the partners who refuses to give up his iPhone and thus iCloud. So every time we schedule server maintenance or an on-call shift, there’s always someone who doesn’t receive the event correctly or sees the time off by… like +1 hour or -1 hour, probably due to a misconfigured time zone somewhere. I tried Zapier about two months ago, but it cost an arm and a leg for what we actually needed it for. Does anyone have a more lightweight solution, whether it’s a SaaS tool or something that can be installed directly on the server? I’d love to hear what you guys are actually doing.