The diode idea is also interesting. Do you think any handy general purpose diode a person happens to have around would be suitable for this purpose? Or is the 1N4148 particularly attractive for this use because of some characteristic it has?
Are any of the on-board resistors of a typical HC-05 or HC-06 backboard important to have in series with the diode, in case we want to use a bare HC-05 or HC-06 module without a backboard?
In those articles there is everything You may want to know about digital voltage level shifting.