How can I use Interrupts to go to Parametrized Pickup Stations?

[Tertius]

The issue is, I can't get to think of how to send mixed type trains requester signals

This is where everyone fails, because the Factorio engine doesn't really support this. Especially it doesn't support keeping a depot of empty trains, and if demand arises sending an empty train to a loading station, let it load something, then send it to the unloading station with the demand. That's not the intended train workflow within the engine.
So don't design for this.

The intended design is you load trains with items as soon as items are available. Full trains wait either at their loading station or in a waiting area/depot made for full trains. The waiting trains already have their destination via interrupt: it's the unloading station that matches the material in the train in case the train is designed for carrying just one material. And for trains designed to carry a set of materials, it's the universal unloading station name that expects the given set of materials.

Then, as soon as an unloading station opens up, a full train that matches the unloading station name will directly proceed to unload.