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Simple Contacts works well.
I use DAVx⁵ to sync contacts and calendar, and just use the GrapheneOS contact app. It's been perfectly reliable with my FastMail account.
DavX syncs with your calendar provider. Any calendar app which uses your calendar provider will be able to your synced calendars
If you just want contacts, there are a bunch of free Nextcloud services.
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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