BetterDisplay

If macOS offers your external monitor exactly one usable resolution and text looks soft in it, BetterDisplay is the fix. It creates custom HiDPI modes so a 4K or 1440p panel renders at a sharp scaled resolution the system will not propose on its own, and it puts brightness, contrast and volume for external displays back under keyboard control over DDC.

The rest is the long tail of multi-monitor annoyance: dummy displays so a headless Mac mini still has something to share, virtual displays as extra desk space, picture-in-picture mirroring of one screen into a floating window you can park in a corner.

DDC is the part to check before paying. It depends on the monitor, the cable and whatever dock sits in between, and the failure is silent rather than loud. The free tier covers enough for personal use that you can find out whether your hardware cooperates before spending anything.

On the licence, the developer says plainly that Pro is not a subscription and the versions you unlocked stay yours. What it is not is updates forever. Around a year in, the next major version becomes a separate purchase, and sitting on an old build through a macOS release is the usual gamble with display plumbing. If you use the Mac for work, note that business use requires a licence even for the free features.