I recently decided to use my google calendar to track my mood, as part of my CBT therapy.
What baffles me is that, there's no way in the web app to assign a default calendar for new events. When I create a new event on the web, I have to scroll through all the calendars that I have and choose the one that I want every time. Meanwhile, in the mobile app, I can simply tap on a new block, the event window appears, and the selected calendar defaults to what I used for my last added event.
You might like Daylio. It's free, local-first (with optional backups to your own cloud or device), and has a good UX.
I think the bigger issue there is that you're trusting Google with your health data. Surely there's an offline calendar app that could help you track it just as well. Anyway, I hope you benefit from the therapy regardless of how you decide to track it.