Enable HiDPI on 2K Displays with M1 Macs Using BetterDummy

BetterDummy

Dummy Display for Apple Silicon/Intel Macs to Have Custom HiDPI Resolutions - an app from one of the makers of MonitorControl.

About

M1 Macs often struggle with custom resolutions. They notoriously don't allow sub-4K displays to use HiDPI ("Retina") resolutions, even though a 24" QHD 1440p display would greatly benefit from a 1920x1080 HiDPI mode.

To work around this, some people buy a 4K HDMI dummy dongle to trick macOS into thinking a 4K display is connected, then mirror it to their actual monitor. Others mirror their MacBook's built-in screen. These approaches have drawbacks and can't solve all problems.

BetterDummy creates a virtual dummy display that you can use as a mirror main, solving these issues.

Advantages of BetterDummy over a physical 4K HDMI dummy plug or mirroring your internal display:

Your HDMI port remains free for another display on Mac Mini.
Your internal screen stays available as extended space on a MacBook (or you can use clamshell mode).
No jittery mouse cursor issues common with physical dummies.
Offers a much wider variety of HiDPI and standard resolutions.
Works with all aspect ratios, independent of the dummy's EDID/firmware.
Doesn't waste graphics hardware, so it's somewhat faster.
Available instantly.
It's free (donations humbly accepted). :)

Other possible uses:

Useful if you're not satisfied with macOS's default HiDPI resolutions, especially on wide or ultra-wide displays.
Use headless Macs (servers) with any resolution and HiDPI mode for remote access (tested on Intel Mac with Intel UHD 630; note the app runs in user space after login).
Scale Sidecar resolutions.
Better quality zooming (System Preferences » Accessibility » Zoom) or high-quality screenshots even on Full HD displays.
Use it instead of or alongside other apps that create custom native resolutions.

Usage

Start the app.
In the app menu, choose Create New Dummy and select your desired aspect ratio.

In System Preferences → Displays, you'll see the new dummy display (e.g., Dummy 16:9).
Activate mirroring. The Main display should be the dummy display.
Set the dummy display as Optimize for.
Set Resolution as Scaled — hold the Option key while clicking Scaled for a fuller list of resolutions!
You can also click Show all resolutions for even more options.

Select the desired mode.

Notes:

This tutorial was compiled on an Apple Silicon Mac running macOS Monterey (the app works on Big Sur and Intel too, but steps differ slightly).
For most configurations, HiDPI 'Retina' resolutions appear by default, and additional non-HiDPI resolutions are marked with a (low resolution) tag when Show all resolutions is toggled. On some configurations, HiDPI modes may be tagged (HiDPI) and standard resolutions appear without a tag.
You might need to fight macOS Monterey's new Displays tab a bit, as the Optimize for setting sometimes resets to the physical display randomly. If that happens, set it back to the dummy.
Enable System Preferences » Notifications & Focus » Allow Notifications » When mirroring or sharing the display to allow notifications when mirroring is on.

The app saves the dummy display configuration and automatically restores it on next restart.

Installation

Download the latest release.
Move the app to Applications.
Start the app.
Use the menu bar item to interact.

Compatibility

Compatible with all M1 Macs running macOS Monterey (MacBook Air, Mini, 2020 and 2021 MacBook Pros).
Also compatible with Intel and macOS Big Sur (limited testing on a single Intel Mac with Intel UHD 630 running Big Sur).
Compatible with headless mode (tested on Intel).