Most Reliably Sync Setup

I sync all the files in ~/Documents via Nextcloud with three Macs running macOS 10.13, 10.15 and 11.0, and additionally with a PC running Windows 10 by Nextcloud Desktop Client. At the moment at the Mac which is running 10.15 I have installed DT3 and having a database which has being indexed all files in ~/Documents. Now, I want to sync and keep being synced this database between both the Mac sunning macOS 10.15 and 11. And possibly by future I want to have access to this database via DTGO via iPhone.

What is the most reliable way to keep this database being synced: iCloud legacy, iCloud CloudKit or – my favourite – Nextcloud?

Personally speaking: none of the above.

The most reliable sync method in my experience is using a NAS on a LAN, as it excludes any issues with internet connections or online services and allows me to fully control the sync store. That said, there can be reasons to use an online service of course.

The question thus is: what are your requirements or criteria that should be taken into account when discussing the ‘best’ sync method. And that should immediately make clear there actually is no ‘best’ method.

And following up on @Solar-Glare’s comments, we have provided a lot of discussion in the Help / manual and the forums. For example…