As I keep a very small stable of ships, the lack of Supercaps and PI's has never been a problem for me, so I never have reason to complain myself. Doing nothing but combat, I am always drowning in manufactured mats and my home station has a manufactured mats trader to boot.
Only agreeing that finding these mats could certainly be an annoying problem for anyone who does a lot of engineering. Ideally there should be a way to go about acquiring any needed item in a game, without having to resort to gathering other things and trading for it.
I guess it depends on what we're calling a lot of engineering really, I have 16 ships fully engineered for specific roles and quite a few swap-out weapons and other gear in storage. I keep getting this awful feeling that I accidentally sold the grandfathered class 5 shield that could fit on my Cutter thanks to a secondary roll that upped the optimal mass, I really must check that one day.
For me at least, it has never been easier to acquire the materials I need. I wouldn't have touched material traders before the ability to get all the HGEs in a system visible from a nav beacon scan because they're best used for trading across or down, never up, and you could spend half an hour or more just circling in supercruise waiting for RNGesus to smile on you and spawn an HGE in the old days. Now with the ability to just drop at the nav beacon in a high population system in Boom state, do a 20 second scan and fly straight to them, the time taken is usually less than it would take me to even find one of the rarer government and state combos on the galmap. In particular for pharma isolators I haven't even bothered looking for a system to get them from since they introduced HGEs being visible from a nav beacon scan, I've just grabbed whatever old crap I can get from the closest HGE and traded for them. Same for military supercaps actually, although they're one of the things I need less frequently.
You're right that the potential range of materials available from missions is limited and I would like to see that addressed but there's a problem with it, in that as soon as you make say 10 more materials potentially available that way, you're massively diluting the chance of any one specific material being available for a particular mission type at a particular station. The only ways around that would be a) increase the number of missions available at any one time, which I'd love (it's nuts that I can check the board at a system with a population of 3 billion and only find about 30 jobs that need doing) but which we know the game just can't handle from when they've tried doing it in the past, or b) give the player a choice of what materials they want for their reward from a set of maybe five different ones. That's probably
feasible but I don't think FDev see a need for it. I could be wrong though, after all they did listen and introduce the different reward types we have now where we can go all in for influence or materials in exchange for a reduced credit reward.