You can find people smoking in the streets too, and that doesn’t make that less dangerous. Google Play app installed in
/system has nearly-root permissions and can (and will) overwrite your microG setup.
Google apps are tightly integrated on purpose to spy on you and make it difficult for you to escape their control, but you are able to do that and more without losing any data. They appeal to fear too and to your emotions when they imply you’ll lost security by stating “allow less secure apps to access your account”. That’s not true. Having all your data in Google servers is a known security issue called “single point of faliure”. Don’t be fooled by them.
You can setup the calendar with DAVdroid as I linked above. The stock Calendar app does the same task Google Calendar does.
Any other E-mail client should be able to do the same tasks as Google Mail too.
I (and other in this forum) can help you with all this setup.