Any way to change resolution dimensions to custom (not listed in Display Settings)?

I have an iMac Retina Display 27-inch. The default resolution size is 5120 x 2800. I also have an HP M27ha 27 inch external monitor, and the default max resolution on that is 1920 x 1080.

Because of the difference in resolution sizes, no matter what I do in Display settings, I can't get a combination of resolution options to match up, so my windows are always significantly bigger when they're displayed on the HP monitor. This really wouldn't be anything to fuss over for most people, but I have OCD and it's been driving me absolutely insane.

Here you can see the difference in size between the two displays when on "default for display".

There's no way for me to adjust my HP display to make it bigger (or make the windows appear smaller) in display settings, as 1920 x 1080p is the largest size. But the thing thats most frustrating to me is when I'm trying to adjust my iMac display resolution, I only have certain options to pick from, none of them being 1920 x 1080 for whatever reason?

The options I have under "scaled" for my iMac resolution are 3200 x 1800 (More Space), 2880 x 1620, 2560 x 1440 (Default), 2048 x 1152, and 1600 x 900 (Larger Text). None of these match up to my HP monitor. One would think 1920 x 1080 would be an option for the iMac, considering it's the most standard resolution size?

So I'm wondering if anyone here has struggled with this as much as I have, and may know a workaround to customize resolution dimensions (either for the iMac display OR the HP monitor) outside of those provided in Display settings on Mac.