So I’m not sure how familiar you are with Unreal’s module structure, but for editor builds, we actually build every module as a DLL and load them dynamically at runtime. You can read a little more about this here
It sounds like you might get a lot more mileage by simply compiling your external DLL as part of the unreal build chain by porting it to a module. That is of course if you actually have control over your external DLL, and it’s not some third party thing?