Well, if you only ever have one window open at one time, then you can do this with ease.
Remember, today most applications are SPA (what we call single page applications). We don't use (nor have) very often a main applcation window, and THEN a child window. For one, they tend to not be touch friendly, and it is VERY hard to move a window around with touch.
but, even desktop browsers even use some "kind" of tabbed interface. And so do most desktop applications. And by tabbed - I don't necessary mean across the top. This access screen shows a tab system from the left side:

So, Access now has a option to use tabbed interface.
And if you :
do NOT use pop windows.
Set access to use tabbed interface, eg this:

Then un-check the display tabs.
If you do above, then you WILL NEVER EVER see the access background window.
You see this:

However, keep this in mind:
The form you launch will re-size to the main size of the Access window - not the other way around.
All forms can't be popup
You are "SPA" like applcation.
So, most accounting and most desktop applications now work this way. And WHEN you launch a form, the window does not re-size tot he form, but remains the current applcation window size.
So, without ANY specials code, you can 100% hide the access background window.
And if you re-size the access application window, then the form will re-size to fill out the current form.
This might look quite bad in some cases - such as this:

Now for above, I on purpose made the form background green - just to PROVE and show that that Access background window NEVER will display with above options.
So, the main issue here?
You can 100% hide the access background window. And you can now do this WITHOUT specials code. Just a few settings.
On startup, you can hide the ribbon - or build a custom one - again your choice.
But, your forms MUST NOT be popup forms anymore.
So, hiding the access background window is very easy - it then becomes a question of form size - size forms re-size to application window size - not the other way around.
On the other hand, if you adopt a tabbed interface (accross the top or left side), then your main window is always in display, and you are in effect swapping out the form (a sub form) for display of that given form. You can write your own code, or use a new navigation form - which does this for you).
If you need to launch separate windows, then you could launch them as popup from this main form - but that main form will of course always hide the access background if you follow above.
If you wish to hide the ribbon, then on your main form - load event, you can add this code:
DoCmd.ShowToolbar "Ribbon", acToolbarNo
So, your main form (which hides access background) could be tabbed inteface (SPA) or it could be a launcher form, and each option clicked on could launch a form as popup - which can be dragged anywhere on your computer - including to the 2nd monitor if you wish.
So, either forms stay always in the main applcation window - but never see the background. Or you use above, and launch forms as popup.