I've never calculated credits per hour, so I have no idea what would be the best number.
For me the most important thing would be to have enough options to spend money on.
Ideally, there would be so many ships and equipment to buy in game, that I would never run out of goals to pursue.
I really like engineering. It does give me goals to gradually improve my ship.
In my alt account I've got A rated DBS after a week of casual playing. I think it's little bit too fast, but what the hell - I could aim for FDL and this will take longer.
Since I pretty much decided I'm going to stick to DBS for now, without engineering all I can do is to find something to accupy myself with, but the game in ship customization would be over. With engineering I can travel to some weird specialist, decide what to upgrade, go and look for more materials, upgrade again, etc. It's a whole game revolving around that aspect. Just getting ship and buying better equipment versions is not enough, but I would be ok with engineering providing only slight boosts and being cosmetic for the most part (like f.ex tuning your engine would give it slightly more power, but also add some visuals on the ship itself - some cables, tubes and coils sticking out of the hull- kind of engineering specific ship kits that you gain through gameplay)
There's a reason why in many games, like RPGs, there's whole gameplay revolving around getting better equipment, constantly pursuing something new, often with elements of crafting and customization. It is simply fun.