You do this by creating multiple accounts within Simple Calendar. On the calendar app, you should (by default) already have access to a local account on your phone, where you can typically add things. There is an option on the Edit Event screen called 'Store Locally Only'. You set this on each calendar entry you want to make local. Sync is not involved. Change the color to something that indicates to you that it is a local entry.
On the other hand, your DavX5 account will be entirely different, here you can add entries which can be synced externally. This will not interfere with your local account entries. The calendar app will allow you to view both accounts simultaneously, usually differentiated by a change in the color code. Use one color for local and another for DavX5. This is how I use it.
Some examples of external CALDAV and CARDDAV servers include Nextcloud, Outlook, Sabre/Fruux and or a web hosted domain that you use which has those servers as part of their services package to you.
Some others like Protomail have calendars and contacts but do not offer CALDAV or CARDDAV.
Good luck. 👍