About the myth that all the nebulae are picked clean, which showed up here as well: yeah, nope. As proof, I'd like to submit the
IGAU Inverness's
nebula survey expedition (EDSM expedition page
here), which went from this June to the end of August, and we've discovered literally thousands of new systems, somewhere around ten thousand, the vast majority being mass code D, around various nebulae. This was done with just a dozen active participants, and we didn't stop because we ran out of systems. (We did have a timeframe, a secondary mission to complete, and then a new addition: deploying the carrier after the expedition to the DSSA.)
The key is to pick your targets wisely. Sure, the real nebulae close to Sol have been picked mostly clean, as well as the more distant ones which only have a few hundred systems around them. But in places out of the way where star density is higher, you can still find lots of undiscovered systems around nebulae. Or well, some lesser known (or less popular) ones too: back a year ago, I found plenty of undiscovered systems in NGC 1931, and not just boring dwarf stars, but all of the main sequence up to A too. Got myself a new ELW and a new AW+WW binary pair there as well.
Whether Frontier had hidden stuff in any of those undiscovered systems is a good question. The NSPs they placed around large nebulae tend to be easy to find, so I'm fairly certain there are no new types to be found there, and the distant Guardian ruins are around less populated clouds. But if you'd like your own systems with some gorgeous views, well, there are still plenty of those around - you just have to do some research first. Or, now that I think about it, you could also just follow the waypoints of our expedition.
There are most likely more such nebulae as well; we originally planned to have more waypoints picked out, but ended up going to less due to time and tritium constraints. There ought to be plenty more fresh systems.