The DMs Guide treasure tables, despite the randomness and multiplicity obfuscating it, result in a defacto official economy at each tier. The official adventures reinforce these economic expectations.
Yes, and this is not a given in a particular ruleset. How DND currently does it is not something we need to assume will still be the case, especially in the context of a rules update to the game (of which economy overhauls is precisely the type of thing that should be happening)
Not sure where you are going with this. A background as a Miner, makes sense to boost Strength. Not sure how that relates to the rest of the level advancement of an adventurer.
I'm using my own RPG to illustrate the ideas I'm conveying about integrating the gameplay loops. It pays dividends to engage with crafting because that loops back into your ability to be good at Combat twice over; first in the items you create and second in the Stats you get for leveling up your skills. As of this writing, maxing out your Mining skill would give you an effective +7 on its own to your Strength, meaning if you trained nothing else you'd have +7 to your Wrestling, Heavy Weapons and Heavy Armor skills. Max those, and you get up to a cumulative +30.
And then by integrating those skills alongside those associated with Dexterity (Stealth, Light Weapons, Light Armor, and Sleight of Hand) and/or Endurance (Conditioning, Animal Handling, Athletics, Smithing), you start building up your Energies, which for a character interested in these would be mainly your Composure (HP) and your Stamina (SM). Increasing both of these not only gives you more resources to spend on Combat, but also makes your abilities stronger because your current Energies (IE, you're current HP or current Stamina) are what you base your Saving throws on, so having them high is not only good because you can do more, but good because you do more with what you do more.
And by engaging with all of those skills, you're now engaging with quite literally every "Pillar" in the game. You're not just a one-note character that only does what those in the Ivory Tower deemed to give you, you're able to do quite a lot across a lot of different scenarios.
And the best part is,
you don't have to engage with anything you don't actually want to engage with, but you are encouraged to give things a try for the same simple reason you don't have to engage with them in the first place. You can shift your Skill points around (with relatively little cost to time or gold), which not only lets you take random scraggler points (like say you had to roll, lets say, Tactics, for some reason and thats not what you're wanting to focus on, and yet you now have a skill point in it. Spend some time, and you can shift that Skill point over to a skill you actually want to use) and assign them elsewhere, but also lets you even shift focus if you don't want to engage with something, or just want to change.
Its a very flexible and ultimately forgiving system. You are directly allowed to get as much optimization out of it as you care to have because I'm going out of my way to give you ways to not have to be optimized, and not have to spend precious cognitive power on engaging with everything in the game, and to not have to spend an egregious amount of cognitive power on these things even when you do decide to engage with them.
I know my game is a very substantive cognitive shift in how TTRPG's are thought of, but one should try to keep an open mind.
There is no minimalist answer that's going to solve all of these problems in one sentence, and too many (not necessarily you) seem obsessed with trying to find a proverbial White Whale.
You mean monsters attacking while mining? Or do you mean statting mining challenges with monster stats?
Yes. And also other things. A simple example is Rock Monsters; have to break them with a pickaxe to actually put them down, and you can then take that basic conceit and design what you might call a "Skilling Boss", to borrow a phrase from video games.
The same basic Energies that feed into Combat are also just as relevant whilst using your Skills; having a high Composure and Stamina is going to be important for Mining and Smithing even if you're not facing some bonkers Rock Monster that you need to carve a hole into so you can stab its fleshy molten core.
As simple as possible but not simpler, is the best game design. Elegance is a virtue. Minimalism that is versatile for various situations, and that is enough mechanically to actualize a narrative in a satisfying way, is ideal.
As simple as
necessary, is what you should say. You can go too far in the other direction, and 5e is actually littered with examples of why chasing minimalism isn't a good idea.