I've been using a markdown-based calendar in Obsidian together with a regular Google Calendar, and so far, it's been working great for me. For markdown, I simply create interconnected notes for years, months, weeks, and days and note down goals, tasks, birthdays, and whatever happened that day (or needs to happen). For scheduling, I use regular Google Calendar events as you want to keep the benefits of invites and all.

A preview can be seen here https://github.com/lorey/obsi/tree/master/example/output/cal....

I've also created a script to automatically generate all the files for me: https://github.com/lorey/obsi