I came here to recommend this. A few notes:

1. There is a CalDav/CardDav bridge which lets you use apps that don't implement the ETE protocol which is good because...

2. The iOS app is nigh unusable. It is painfully slow, requires manual syncing, and if your screen turns off in the middle of a sync, it may get killed by iOS forcing you to start the whole thing over again. Just set up the DAV bridge and use iOS's native DAV syncing.

Seconded. It was a bit more awkward to set up compared to Nextcloud or Radicale (which both support standard DAV) but the Etesync Android client and desktop bridge work very reliably in my experience, and the added bonus is end-to-end encryption. I now have my contacts and calendars on Etesync and haven't needed to touch it, easily configuring it on new client devices as needed.