Making stuff readily available from missions only is a bad idea, there should be alternatives for any level of player.
It doesn't take
that difficult a mission to get G5 material rewards available as an option. The basic "scan this surface data point" ones can do it, as can hauling missions completable in a single run by a Krait/T-7 type medium ship, or two runs in a T-6. And there aren't many key non-combat blueprints which use
manufactured as their G5 component as it is.
But I
don't think they need G5 material acquisition to be easy, either - in fact, it'd be more "conventional" game design if it wasn't! - after all, you can get a fully-G4 engineered ship (by definition) without picking up a single G5 material, and that then gives you the capabilities to take on pretty much any challenge in the game anyway for picking up the G5s. Encouraging breadth-first rather than depth-first engineering is (see below conversation with Helmut) one of the things that the current design doesn't help with, I think [1].
[1] It's probably too late to change, but what I think they should do is redo it so that "common knowledge" engineers have all the G1-2 blueprints, the next tier up have all the G3-4 blueprints, and the finale engineers like Sarge and Jameson and Palin (and the Colonia ones) have all the G5 blueprints. That'd encourage people to start with the easy high-impact blueprints
and stop the weirdness where Jameson has some of the toughest unlock requirements in the game ... for some marginal improvements to niche modules.
Maybe figuring this out is the touring test of the game

. In all honesty, this is pretty obvious - makes me wonder if all those who complain about the grind of G5 engineering ever look at the numbers.
Exponential costs for at-best linear performance improvement is the game style even before engineers (look at those A-rated fuel scoop prices, even
with today's inflated credit earning rates you'd rarely actually make back the money spent on an A-rated scoop in time saved later, unless you're going on a very long honk-scoop-jump journey)
I recently had a passionate discussion on the forums about shield engineering, if you remember

. During that I did some engineering maths and found (again) how far G3 actually gets you - engineering really is the embodiment of diminishing returns.
Shortly after the engineers were added to Colonia, I did an experiment of building a ship
purely with what you could get locally - so B/C-rated components engineered up to mostly G3 max. The difference in performance in PvE [1] between that Krait and its A-rated G5 equivalent was measurable on paper and almost entirely irrelevant in actual flight.
[1] Though I successfully ran away from a couple of PvP attacks in it too.