In terms of listening (not playing), I have a metal / hardcore background, too. Yeah, split releases are big there, esp. on the hardcore side of things.
I don’t know what works best, but this is what worked for me:
Earlier this year I wanted to do a collaborative bandcamp EP covering a variety of styles, so I directly messaged musicians who I thought might a good fit for the project. If I remember correctly, 5 of the collaborators were from here on LINES (Disquiet Junto) and the other 5 were from soundcloud and elsewhere. It was a lot of hard work logistically - matching the right tracks with the right people and working to a (self-imposed) deadline - but it was deeply rewarding. Every collaborator brought their own distinct style to things and made the finished album way better than it would have been otherwise. I sent my brother unlabeled demos of the finished tracks, and the five or six he thought were the best were ALL collaborative tracks!!
I only had to send eleven emails before I found ten contributors. Unfortunately, I’ve only met one of them in real life, and I’m not even sure how many of us might even live in the same country. Technology is pretty amazing…
Just approach people. The worst they can say is no thank you!!