Collaborations are all about making connections and getting to know more producers, vocalists, and songwriters to work with. You can join a producer or artist forum, for example on Discord, and make connect with musicians working with the same genre as you do. You can also get yourself creative and find people outside your comfort-zone to make something in a completely different genre!
You might think you'll get a reply faster if you send 15 emails with the same track to the same label. But the only thing you're then doing is creating a backlog for both all other existing demos and your own. It's much better to let the record label take their time and then when you get a response be thankful for that, also do respect the label's decision.