@ashpool
Thank you for all your help.
I ended up settling on Mode 51 (the largest 4:3) resolution the monitor will accept, and choosing a 4:3 Theme. This makes is look "close enough" for the ES look/feel. Because this will be all MAME games, I have to set the DMT for each one, so luckily any changes to ES doesn't affect the RetroArch settings.
As for the Screensaver and Splash screen (currently a static image), is I will over-squish them in an image editing program so that when they are displayed on the 4:3 setting, they will look like the correct aspect ratio. At least that is the plan.
Thank you all for the help, support, and guidance! My first build and so many online tutorials make it look SOOO easy, yet the online community has been there with support, tricks and tips, and overall welcoming acceptance of noobs. Thanks again!