As I developed my fan game and made the decision to use this Mechanic interaction with a Pokemon's Primary Type, I have grown to really like this mechanic. It pairs up nicely with the Tera mechanic, and when used with Universal TM's, it grants every Pokemon a consistent and reliable STAB attack, without having to make custom moves. This mechanic however, pretty much requires extensive work on re balancing Pokemon's metagame. A mechanic like this in Vanilla Gen 9 Pokemon, will cause an astronomical amount of issues, given that it grants everything access to a Strong STAB move, depending on how their Primary and Secondary Type is arranged.
This is a feature that someone can't just slap onto a fan game project, without evaluating the implications it would cause to the metagame balance. But if someone decides this feature is what their fan game needs, then it's a mechanic that gives more to think about that is a lot of fun.
