BetterDummy allows you to use HiDPI ("Retina") on sub-4K display M1 Macs.

For example, a 24" QHD 1440p display would greatly benefit from having a 1920x1080 HiDPI "Retina" mode available - now you can.

To circumvent that type of issue, you could buy a 4K HDMI dummy dongle to spoof the macOS into thinking a 4K display is connected and then mirror the contents of this dummy display to the actual monitor to have HiDPI resolutions available. You could also utilize the built-in MacBooks screens to mirror the external display. These approaches are not the most efficient and have potential issues.

BetterDummy was designed for such situations; it creates a virtual dummy display which you can then utilize as a main mirror display.

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

Your HDMI port will remain usable for another display on the Mac Mini

Your internal screen will be available as a vast space on a MacBook (or you can use clamshell mode)

Will not suffer from issues that are prevalent with the physical dummy (like jittery mouse cursor)

Offers a much wider variety of HiDPI and standard resolutions

Works with all aspect ratios does not depend on what resolutions get recorded in the dummy's EDID/firmware

It does not over-utilize graphics hardware, making it somewhat faster

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 (for example, Dummy 16:9)

Activate mirroring. The Main display should be the Dummy display

Set the Dummy display as Optimize for

Set Resolution as Scaled - you should hold the Option key while clicking on the Scaled option for a fuller list of resolutions

You can also click Show all resolutions for even more resolutions

Select the desired mode

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

Similar:

How to Check or Change Your Screen Display Resolution