Move-SPUser and stsadm -o migrateuser are farm-wide operations. There is no way to restrict identity migration to a single Web Application.
There are 3rd party products that can accomplish this, like Metalogix ControlPoint can clone permissions within a specific scope.
We could enable and use IE Mode in Microsoft Edge to make the open with explorer works again.
Try following steps to enable it:
Open Microsoft Edge.
Click the Settings and More (ellipsis) button on the top-right
Select the Settings option.
Click on Default browser.
Under the “Internet Explorer compatibility” section, turn on the
“Allow sites to ...
Provided you have a load balancer in front of your FEs, yes I think that would be the right approach -- with a farm that otherwise has no high availability, there's no reason to separate out DC onto a dedicated server as your environment isn't large enough to call for it.
You could make your farm highly available by making the backends App + Search, though ...
Here is an introduction to SharePoint Server in Microsoft's official documents. Many features are interlinked, you can view the features you know, and note that the applicable version is indicated in the document：
Getting started with SharePoint Server
In addition, here is a simple video for beginners, if you are interested:
Microsoft SharePoint 2019 - Full ...