2) What's the reason for having a huge galaxy, if it's completely empty?
3) Has this been play-tested? What did the testers have to say about the long cruise times?
4) Do you really think this is in any way "fun"?
2: being able to stamp your name on stars I guess, probably that's about it. It IS indeed impressive, one of the main reasons I wanted to play this game, but yes it becomes quickily evident that you'll never gonna see even 1% of them (that'd be ~4 billion systems, I'm at ~1k current)
3: yup, I remember many voices hinting that flying in a straight line for 10min in supercruise for some thousand lightseconds *might* get boring after a while, like after the 3rd time. No damns were given.
4: sometimes (for me atleast), but more of the time am I just trying to find something and have the opposite of fun.
Also about your mission problem: you're not the only one who's BB get's spammed with untakeable missions, I'm maybe 2/3's to Elite rank on the 3 types and I also get lots of missions for which I am either too low ranked by atleast 1 or multiple ranks, not friendly enough with them, or both.
It's a pain having a mission completely within your capabilies to be generated and the game just going "NO U".
Another tip I'd give you is to avoid combat until you got familiar with it, like when someone interdicts you in supercruise you can target them (if you bound "Select highest threat" to a key that is) and see if they have a bounty and whenever it's a player or not though the latter is shown on the radar, players are hollow, npcs not.
So anyways if you get interdicted and it's something like a Viper/Eagle or Sidey and you're in your Eagle and armed (I'd recommend gimbal'd pulse lasers but that's up to taste) then you can just cut the throttle and submit to the interdiction, it should then only be a fight between you and him where you can easily try fighting.
Well that or the combat tutorials but I think this here is more practical, also you can get moneys, "wanted" means automatically money when killed, though NEVER attack until the first targetting scan is done (it'll either show CLEAN or WANTED), even IF the target is wanted, you will become so too if you shoot first, except in anarchy systems.
And even if you are a bit more confident with combat I'd still recommend to not take assassination missions (the ones with the big crosshair and a named target) because you never know what you get, could be an Asp, a Python, or worst, an Anaconda.
"Kill x Pirates in y" on the other hand can be easy though time consuming as they can take ages to spawn at the nav beacon for example or which is probably a better place, an extraction site.
Aside this I can only tell you to just look around, try things out (but carefully, ALWAYS keep money at hand for atleast 3 ship rebuy's, just in case) and see what starts to grab your interest.
Maybe mining is something for you, or trading, maybe bounty hunting, or pirating npcs (or players) or just going out there trying to stamp your name on things.
I'm still somewhat looking but so far I like smuggling, gives a good amount of money for they trouble, way more worth it than manually buying cargo and bringing it from a to b or legal trading missions.
If you wanna try, get something quick and nimble, or stick to outposts, no scans there. Viper / Cobra are probably a good start, Hauler too probably, later T6 and Asp if you get their handling down, can be a bit trickier to get past the scans at stations but totally doable with the big Asp.
Another nice thing about smuggling is also that you can with enough cargo space stock up on missions by waiting a bit when docked, every 5min there'll be some new missions, and old ones gone (bottom ones go away first, top ones are the newest) and by that and quite a bit of luck I once had a run for around 8 missions, most smuggling, ALL to the same station, bringing me nearly a million in a run xD
Dunno anymore though how long I kept stocking up on them, might have been sitting there a while watching a bit of YT or so. Still nice to see the whole first BB page being turn-ins.
Oh and they can also help with reputation for the big 3 groups, Federation, Empire and Alliance, just check the mission's giving's group allegiance on the right panel (first tab once to the right, then you get a system faction list with the allegiance icons), by that you can focus on missions that increase your reputation with these 3 too when finishing them.
I've smuggled half of my friendly ranking with the alliance that way xD