Hello,
I am looking for a mobile application for Android that is compatible with Nextcloud, of course, and Google calendar. At the moment I have Etar, but this application rings at any time and seems to be buggy. The interface is nice, but I can’t stand Etar’s alarms even if the appointment has passed, or I’ve deleted the notifications.
Thank you.
//EDIT : I use DAVx5 for synchronisation.
I’m using opensync and have all my contacts and calender synced with my cloud and I use google calender but without google interference ofcource.
I use Simple Calendar Pro on GrapheneOS (de-googled Android without play store) Choices are somewhat limited when you cannot use Play services. 
If you have a phone with stock ROM and Google Play services enabled, you can use most of the apps that are available in the Play Store. Most calendar apps are really just front ends for the Android Calendar Provider API which DAVx5 also uses. So you should be able to use the stock calendar app of your phone or the Google calendar app as @Vincent_Stans already stated.
If you want something more customizeable and configurable I highly recommand to pay a few bucks and buy Business Calendar Pro. That’s what I used for years before I switched to GarapheneOS. And to be honest none of the (two?) apps in F-Droid come close to it. But I guess a few sacrefices have to be made if you want to de-google your life