Karabiner-Elements

Karabiner-Elements sits below the application layer, which is exactly why it can do things a hotkey utility cannot. It sees a key event before any app does, so Caps Lock can become a hyper key, one physical key can behave differently tapped than held, and a cheap external keyboard can get its own remapping while the built-in one stays untouched. A great many elaborate Mac keyboard setups have this as their foundation.

Getting there involves JSON. The preferences window handles straight swaps, but anything worth having is a complex modification, which you either import from the community rules list or write yourself in the config file. There is no rule builder and probably never will be one.

The licence is the Unlicense, a public-domain dedication rather than merely a free download, so nothing can expire and there is no vendor to outlive. The developer accepts optional sponsorship and asks for nothing else. The real cost is a driver extension you must approve in System Settings, plus the habit of checking for an update before installing a new major macOS version, because a tool that operates this close to the kernel is precisely what a new OS tends to break.

If all you want is Caps Lock remapped to Escape, macOS has done that in Keyboard settings for years.