A hat tip to you that you're looking to change to a different file manager app -- ES was at one time a very highly regarded file manager app, but after it was bought out by a different company a few years ago its shiny features became really tarnished.
Anyway, your request for a simple replacement with a low learning curve is something that's a very subjective point. What's good for someone else isn't necessarily what's going to be good for you. I'd suggest something like this 'Moto File Manager' app, or 'Explorer' app as their default user interfaces have at least a similar 'look and feel' to ES Explorer.
Moto
https://play.google.com/store/apps/details?id=com.lenovo.FileBrowser2
Explorer
https://play.google.com/store/apps/details?id=com.speedsoftware.explorer
But these are just non-specific suggestions. You might want to just go to the Play Store yourself to find something that appeals to you.
https://play.google.com/store/search?q=file manager no ads&c=apps
We can all tout our favorites but this is something you should decide on based on what
you are most comfortable using. Pay a lot of attention to the screen captures each file manager app provides in their respective Play Store page. They will show each app's default user interface and see what appeals to you -- functionally they are more or less equivalent, it's the user interfaces that vary so much (... and the user interface is how you interact with the app).
What phone do you have? If a Samsung model, there's a pre-installed file manager, Samsung's 'My Files' app. It's a pretty good, basic file manager with a clean, user-friendly interface of its own that you might want to try out:
https://play.google.com/store/apps/details?id=com.sec.android.app.myfiles&hl=en_US