The core of this problem is that Elite has a very low skill ceiling when it comes to variable outcomes. Whatever activity you choose to participate in basically has a set rate at which you can expect to earn credits. Most variation in outcome is down to RNG rather than how you play.
It doesn't matter how carefully you plan, how precisely you optimize your ship, or how attentive and skillful you are in executing your chosen activity. High-level play and thoughtful planning will not significantly elevate the rewards you are able to bring in. Most activities can be accomplished with a fairly low level of attention, foresight, and knowledge; and nothing you do beyond that mediocre level of commitment will substantially increase your rewards.
It's not that mining needs to give X amount of credits per hour in order to be a valid activity. It's that there's not sufficient depth of play to allow for depth of outcome. It's fine to have an activity which only gives out modest rewards to the uninitiated and inexperienced; it's another thing entirely for that activity to remain more or less static no matter how much you learn or how skillfully you play.
And just about everything is like this. Without greater mechanical and technical depth in the various activities in the game, there cannot ever be a form of balance other than "set all activities to generate same amount of cr/hr no matter what," which will still be imbalanced and upsetting to people because that means you are now "forced" to engage in whichever activity carries the lowest risk of setbacks, has the lowest barrier to entry with respect to outfitting, and is the most widely available. And then you just end up with people ranting about how the mission board doesn't give out enough of those mission types and doesn't refresh fast enough.
When the Thargoids came, we suddenly had a kind of "boss" enemy which required specialized outfitting, strategic play, and some combination of either higher numbers of ships or incredible skill and optimized outfitting. Thing is, we should have had something which fulfills this role for a long time. There's no reason we couldn't have had high-value assassination targets, with specialized engineering builds and unusual AI patterns, which we could learn to take down using a combination of skill, planning, and coordinating with others. Likewise the surface base assaults were never really developed or integrated into the game very well and could have served a similar purpose.
That's combat, but there ought to be equivalent high challenge opportunities in every area of activity. You give people the opportunity to "git gud" at their chosen activity, and allow for appropriate rewards to follow, and suddenly relative payouts between professions don't matter as much, because there will also be tremendous variation *within* professions, and chances are no matter what you are doing, you could be doing it better for a better reward.