I definitely think you should be able to add keyboard layouts at a software level on iPadOS and iOS, but one possible solution to the problem right now is using a programmable keyboard using QMK [1], ZMK [2], or some other open-source firmware. There's even a usb-usb converter [3] that will convert keystrokes from any keyboard into ones that you map using TMK or QMK, so you can use your own.

[1] https://qmk.fm/

[2] https://zmk.dev/

[3] https://geekhack.org/index.php?topic=69169.0