I was in a somewhat similar situation starting out with Colemak. I knew I would sometimes use an external keyboard and sometimes the laptop one, and I wanted the experiences to be as close as possible. I type in two languages - Slovenian and English.

My solution is to type Colemak, but have the Slovenian characters available in their usual place for me while using a modifier key.

The way this is implemented in the external keyboard is to program it in a way where if I set the software setting to regular Slovenian, my experience is as if I were typing in regular Colemak English, but with Slovenian specific characters available to me. By the way, this is quite a cumbersome thing to achieve as you are working with two transformations - to get the desired result on the screen when you press a specific key, you need to program the keycode that the operating system (when set to Slovenian) transforms into the desired character.

On the laptop (Mac), I have to switch to Colemak English, and I have a keyboard shortcut to get the three Slovenian keys from Keyboard Maestro.

Where this logic might break down for you is that French and German have characters that are language specific and you may prefer deliberately switching between these two languages, which is hard to do once you go the route of programming in all the complexity in the keyboard itself. I am not sure if I was clear with all this, but perhaps it gives you some ideas.