The Mining and Exploration changes improve both of those professions, the latter of which is a core activity in the game.
"Existing core games issues" means different things to different people. Some want core professions like Exploration, Mining, Smuggling, Bounty Hunting, etc improving. Others would prefer to see the later additions like Powerplay, CQC, Multicrew, etc receive focus. Others still would prioritise networking, matchmaking, full crossplay, and PSVR support. There is vocal demand for large BGS improvements and more support for emergent, competitive gameplay. Immersion is high on some players' lists, NPC persistence, and the different game features being more closely tied together.
Different strokes for different folks. There are many features in the game, therefore many areas for FDev to improve.
They're not all that different, you can often find different folks saying the same thing about the same features and areas.
Mining and Exploration are what I'd call the few areas of the game that are actually pretty fine as-is. Yes, improvements to them are *welcome*, and have been called for for *years* during which they've been neglected, but they are not what I'd expected when I heard "Core game fixes and improvements" because, despite the neglect, they are still quite solid.
Combat (consistency and balance, namely; I'm not referring to flight modeling so much here), Engineers, Powerplay, Supercruise (USS baloney + taking a bit too long to traverse), Horizons (which, I mostly mean rock-pew-pew-vacuuming in particular, but when are we gonna get other SRV types and such?), CQC, Superpower Ranking, Multicrew & SLFs - these are all things that on the whole are *not* fine as-is.
(I mean, sure, Supercruise isn't exactly breaking anybody's game but it's something that most folks sort of put up with rather than enjoy. So I think it stands that it could use a lot of improvement. Sort of the same thing going on with Horizons stuff.)
The BGS is definitely a 'core game' item; I'll agree that the BGS, outside of providing basic needs for missions and USS finding, is probably not ideal as-is and could use improvements. Folks more dedicated and smarter than I have posted on the matter considerably in the past couple of years - and of any point of development, by the sounds of what this year was supposed to be, this would have been the time to expect it, no?
Network and matchmaking improvements are things I'd expect to continually receive attention over time - I wouldn't call these "core game" aspects, they're just naturally part of being an online multiplayer game - 'background stuff', as it were.
Full crossplay is a neat dream, but I'm uncertain of its future in the face of Microsoft and Sony - I reckon that ball is in their court at least half the time if not more, and that means time delays, red tape, and so on. If it's being done by other games currently (a topic I'm unaware of), then sure, it's a realizable dream, but like the network & matchmaking improvements, it's an enhancement rather than a core part of the game.
PSVR is pretty important, I'm sure, if you're a PS4 user.

So I'd expect that to be an ongoing side-project sort of deal that hopefully can be accomplished as soon as is practical.
NPC persistence and other new features are also important, but not as important as making sure that this other stuff that's already in the game is solid and able to provide a good, fun foundation for other new things to be attached to. (Warframe, as an example, I feel is making a long-term mistake by not worrying about their foundations as much as they ought to.)