1. Syncing

Method Price Pro Con
Obsidian Sync $4 per month Simple, easy, encrypted, secure. $4 per month
Git / other version control system Free software;
free hosting available
All under your control Learning curve
NextCloud You host at home
or you pay for hosting
All under your control Learning curve
SyncThing Free Peer-to-peer among all your personal devices A bit fiddly to setup
Pick your own file sync tool! Obsidian is text files plus attachments; any sync tool will work
Don't sync your "workspace" file

If you have a vault open on two computers at the same time (you may have left it open and then sent one computer to Sleep), there is one file that can generate a lot of syncing conflicts:

I recommend that you configure your syncing tool to ignore the file .obsidian/workspace.json under the vault's root folder. This file changes every time you navigate to a page in Obsidian, and it doesn't need to be synced across all of your computers.