Hello David @db_Vanir
you need to do the change, you and your users. Yes, it's a process when you migrate your local files into the cloud, but you need to do that, when you wanna use a modern M365 world.
I don't recomment synchronisation, you build redundancies and THIS confuses users even more.
Build an modern architecture and use the tools from Microsoft, like a lot more people.
I'm not agree with you, but here some inputs:
For synchronisation you can use Thirdparty tools and a lot backup software have a M365 integration, yours too ;)
Best and good luck into your modern change ;)