The cost of Windows Server is basically zero with https://github.com/massgravel/Microsoft-Activation-Scripts . Yes. It can be used to activate Windows Server 2022.
Windows Server 2022 is basically another full desktop. There is no comparison to RDP in the Linux world.
The thing missing is Synology's SHR raid. But you can use DrivePool I guess.
All those concerns over resource overhead are useless, unless you want to use a very weak computer. Windows Server is extremely fast with modern day computer.
Because there are computer illiterate people get confused all the time when documents can be saved in the cloud or local, so Microsoft now chooses to make cloud the default location for saving documents. This helps many people, but it can also be dangerous if saving to the cloud fails.
The best way is saving to the local drive and letting OneDrive sync to the cloud. But you need to change settings in OneDrive. It can still be confusing.