Thank you all for the info!
In my case, it was quite a premature optimization case (it turns out that many drawcalls created this way don’t hurt the performance that much + I’ve limited the max decal amount). However, still, it would be nice to have a way to conveniently pass data to decals without creating a material instance for each one.
As a workaround for now (as an addition to your other ideas mentioned above) you can also reuse material instances if it’s possible for your project (more info)