Seafile

Seafile is a file sync platform built around its own storage format, which is a big part of why its sync engine is famously fast and robust with large libraries and many small files.

That custom format is also the main trade-off: files on the server are stored in chunked form, so you access them through Seafile rather than browsing a plain directory tree. The open-source Community Edition covers the essentials; some advanced features live in the Pro edition.

Worth a look when sync performance is the priority and Nextcloud feels like more platform than you need.