How can I make sure texts from friends with iPhones don't go to iMessage instead of my Android phone?

You can not control how the sending phone addresses messaging much from your phone. As you mentioned, you have already deregistered your cell phone from Apple serves so that leads to the conclusion that your contacts are sending to your AppleID, by their choice.

Your only lever on the receive side is to change your AppleID to something no one knows. Then, you won’t have incoming iMessage transmissions by email or by mobile.

• It’s likely not practical to get everyone to change their address book to clear their contact database to suit your preference, each time they make a mistake sending to your AppleID.
• It’s also equally impractical to just block them entirely on Apple devices and hope they change the way they communicate.

But it never hurts to ask nicely, maybe people would change up their conversations instead of reusing the old conversation that is easy rather than checking the “to” closely.