As predicted, you can now get a third-party internal storage upgrade kit for the M4 Mac mini.
The supplier, https://expandmacmini.com, only offers a single product at this time - a 2TB module for the M4 Mac mini (M4 Pro is “coming soon”) for $260.
According to AppleInsider, performance isn’t quite as high as Apple’s SSD, but is still faster than anything you can connect externally. And much less expensive than what Apple charges to have 2TB pre-installed from the factory.
Installation seems pretty straightforward if you’re comfortable with the partial-disassembly necessary to physically install the module. Afterward, you must connect your Mac to another Mac and run Configurator to initialize the device and install macOS.