When you go to do an upgrade, always go with enough mats for 3 rolls. Never accept a bad roll (why you did this i have no idea!) and out of 3 rolls you should get something good.
I did grade 5 dirty drives the other night, i had enough for 4 rolls. First one was good, very good. So accepted it (i always tend to accept any first roll on a clean module anyway as its usually an improvement over stock). Second was meh. Third was even better than the first! I had the fourth sitting there, so thought what the hell and made the roll. Wasn't a good one though.
Basically, most rolls are decent, ignore any bad rolls, and always take enough for at least 3 tries just because you can get a bad one.
If you only have one higher grade upgrade, don't waste it on the ship that already has a decent lower grade upgrade.
My corvette had a decent G4 FSD upgrade (did that pre-simplification, so a lot lot more work to get there for about 35% increase) and I ended up putting the G5 upgrade to my conda, which turned out to be rather mediocre and actually worse than what the Vette had.
The end result however is, that I have
two ships with a significantly improved FSD ranges, which for the Corvette is a pretty decent QoL improvement and for the Conda is nice to have to shave off about 1/6 jumps.
Doing a second upgrade on an already upgraded module doesn't seem to do anything (at least my gut feeling after nearly 200 upgrades now - many of them low level ones just to push trough the first levels of the engineer to get the follow-up one and caused by low level mats literally repeatedly hitting the storage caps, even after they have been raised). It just appears to be "worse", since instead of straight upgrades from stock, you can get 'worse' results, if the blueprints have overlap.
If you do G1 to G4 or G5 upgrades, there is no overlap, either.
As for the epic time investment.
I've done about 5 hours of mining that I would not have done otherwise (which includes the Painite mining) and about 3 hours (?) of planetary surface. All the other junk (including several copies of ultra rare mats like embedded firmware, imperial and core dynamics shielding, exquisite focus crystals), I got by doing missions that offered them.
The annoying part that's still in the game is lugging around those "hard earned" (aka "picked up along the road") commodities, not being able to swap into a ship without cargo space (haven't seen my FAS in months) and getting constant interdiction attempts.