Since you run myNode you will very likely break things when you try to mess with rpcuser and rpcpassword in bitcoin.conf without other modules like the Electrum server knowing about it.
I'm not familiar with myNode, but it should have some central config files that define certain global settings for the modules that you can activate in myNode.
I wonder why the Bitcoin RPC Explorer module needs any manual tinkering in myNode. Shouldn't it run out of the box when enabled?
You will have to try to find such details in the documentation of myNode. As I'm not familiar with myNode, I can't tell, sorry.
I run nodes with RaspiBlitz where it's fairly easy to tinker with config files for optional modules. With nodes that are setup based on RaspiBolt (doesn't need to run on a Raspi 4B or 5) you configure everything yourself (some Linux experience is of great help).
My other Umbrel node has every optional module in docker containers. I didn't tinker much with my Umbrel but as far as I see, most of the config files are exposed to the docker host file system where you can apply changes. Persistant changes are another beast, i.e. changes that should survive a module update or similar. The Umbrel UI doesn't provide many options for this and I guess this is their policy (eye candy is more important than configurability options).
Persistant changes in myNode could also be challenging if you don't understand how myNode is setup internally.