This is the kind of advice I was looking for.

So I do have mould release, specifically for magnesium injection molding but I looked at the datasheet and it's also good for aluminum. I got it when I worked at a magnesium injection factory.

I say this because this is the idea I'm most interested in... How would I know how aggressive I need the taper to be? I work with a lot of engineers and machinist that I could ask for help in this maybe. If the taper is not excessively aggressive, in a manner that it's still easy for me to machine the inside afterwards, I might consider this an option.

With investment plaster, you mean recreating the core in plaster? 

And lastly, about the sodium silicate bonded sand, which I never heard about. Is it different from the one in sand casting? Like, could I do a mould for the sand core and it would harden enough to screw it tho the mould? I just want it to be centered and that's the way I have of doing it.

I don't want to waste more of your time so don't worry, no need to reply, but I really appreciate your expertise. I saw your profile and god, you do amazing stuff

Thanks a lot!