I’m having more success now that I’m being more selective, finding systems with low or no populations, and going to rings that are further from the main stellar body. Apparently other players mining these areas will reduce the chance of you finding fissure asteroids, so it’s best to try and find something out of the way. I’m over in the “western” edge of the bubble, having decent luck finding good sites. I’m getting to the point where I can pull up to a site, and within a few minutes know whether I’m likely to get anything - there are certain types of asteroids and patterns that yield fissures, and when I don’t see any at all I usually leave. They’re rare enough that it’s a real thrill to find one, but no so rare that I give up in frustration. When you find a good system you’ll be cursing not buying a larger cargo bay (or more limpet drones!).
Yeah, ED is a slow burn, it’s not the game I’d suggest if you’re after a 3D combat fix. Brian has the right of it: Overload, and Rebel Galaxy Outlaw. I know you mentioned enjoying Everspace which is the only reason I’m not including that.