ColdFusion 2018 changing docBase renders administrator page grayed out with a search bar.

Daniel, there are a few things to consider.

• First, as for that cfformscriptsrc value you set, it should NOT be an absolute path. It should be a webroot relative path (or an alias). If set as a webroot relative path, it would be pointing to a folder witin that cfusion/wwwroot, so just saying /cf_scripts/scripts would suffice, and that is in fact the default
• Most people get in trouble when they change that (which as you may know, you can do in the CF Admin on the first "settings" page as "Default ScriptSrc Directory", but I realize you could not do that because you could not get the Admin to work at all 🙂, and you were changing the underlying xml file anyway).
• And indeed, if one DOES change that value to anything ELSE than that default, then even if they correct their web server (IIS or Apache) to have a new virtual directory or alias of that name (which should then point to the absolute path for that scripts folder), they often find that their CF Admin fails to work--whether in part (like the Updates page tabs not working) or perhaps in whole (as you experienced).
• The problem is that they need to ALSO change the CF built-in web server configuration (which is used for the Admin by default since CF2016, and optionally before that). It's actually the Tomcat web server, and in fact that is what you were perhaps trying to do when modifying the preresources elements in the server.xml. Where did you get instructions to do what you were doing?
• I will say that Pete Freitag did a post some years ago which covers both newer and older CF (Tomcat) versions. See https://www.petefreitag.com/item/867.cfm. It focuses on the change needed when one wants to define an "alias" in that web server, such as is needed if you do make changes to that CF Admin "default script src" path.
• But we see here that you were having it define the location of the CFIDE folder...and note that is NOT where the scripts folder is, for CF2016 and above. Instead, it's now in the cf_scripts folder, as a sibling to that CFIDE. So instead, as Pete's post suggests, you want to create an alias (a preresource) that maps that default scriptsrc path as you may change it (in the admin, such as "cfscripts") as a webappmount that points to the real cf_scripts/scripts folder. You were close. 🙂 Again, see Pete's post for more.
• Finally, I should note that in all such cases, you can really help yourself in troubleshooting such situations when you can SEE what is amiss, by viewing any browsers "dev tools" (right click on white space in the browser and choose something like "inspect"), where the tool's "network" tab would show (on a refresh of the Admin page) that there were many 404's, and you'd see how they are trying to call back to the server. That may help you see where your mistake is, at least from the client side. 

Let us know how you get on.