Does anyone have or know of some combination of KDE and Android apps which enables synchronizing calendars between the two apps?
You’ll need a sync service for that or host your own CalDAV server (or part of something like Nextcloud groupware).
KDE’s apps can sync with your server but they don’t include a server.
I use DAVx5 on android to sync my Nextcloud Calendar to Google’s calendar app.
or of course you can use Outlook, or Google Calendar etc. that both Android and KDE sync with