I don't know if you've read it, but Expeditious Retreat Press published 20 years ago "A Magical Medieval Society" (Western Europe):
preview.drivethrurpg.com
There's even a third edition of it with more material in it:
preview.drivethrurpg.com
It's an interesting read, covering many of the questions, but also giving many different perspectives of a Lord vs a peasant.
But what people seem to forget is that those coins that you've found in a dungeon is mere change, literally and figuratively. The real wealth is in land and what that land offers. And just buying land as a non-noble... That's going to be expensive! And sir, how did you get all that gold? From the dungeon on
MY land!?!? You Sir, are a thief and a scoundrel! As others have mentioned: taxes! To the Lord, the King, the Church, possibly even the Guild and the City... Also keep in mind that people that have a lot of wealth get to be the target of others out of jealousy and/or greed. And while the Rogue might detect a pickpocket attempt against other members of the party, that Rogue isn't always around...
Pretending to be low level caravan guards might be something the non-lawful might attempt, not telling people they went to the local dungeon or that they eradicated the local bandits... On the other hand, joining the local Adventurers Guild might give you some unionized protection, but you better pay all those dues and taxes, (which the Guild probably negotiated on your behalf) or else!
Magic might be more prevalent then normal, but that doesn't mean that everyone can become an adventurer, especially not a spellcaster. Maybe it's just not those stat requirements, maybe there's something else in the secret sauce, genetics, fate, unlisted stats, etc. So maybe there are a few spellcasters in the village, but would they be allowed to use their mending as a business? What do the Guilds say about that? Maybe all the Tinkers are actual spellcasters that can cast Mending cantrip and have their own Guild (and protections)... Those that can cast Comprehend Languages are translators and those that can cast Cure Wounds are healers... You can cast Unseen Servant as a ritual, when you get going you can cast 6/hour, so 6 unskilled laborers that can unload a wagon, boat/ship, dig, etc. 12sp/day, for a moderate living...
Also keep in mind that if magic is that common, it will also be available to the government, nobility and rich merchants. So that person that can cast Mending might actually be exclusively working for rich people who pay more to be on call to repair their expensive cloths, jewelry, furniture, etc. Suddenly there is work for other folks repairing for the common man for a decent fee. It's all about supply and demand, spellcasters are a limited supply and there is a high demand for their services, thus high prices, that normal folks can't afford. Fantasy medieval society isn't some communist or socialist state where all work for the common good...
Also, most people are just happy to life, rocking the boat might very well mean ending that life... That's also the reason why adventurers are relatively rare. Now if their lives become hopeless, then things might eventually change. But folks working from sun up to sun down, 7 days a week, hard, with 'ok' food and drink tend to not have the bandwidth to plan a rebellion... That's generally the realm of those that don't actually have to work that hard... And the targets of other, more lawful adventurers to squash... Keep in mind that the Lord's job is to also protect the peasantry, so the Lord is also the person that pays the other adventurers to eradicate the Orcs and Goblins that plague the lands. And let's not forget grandpa "In my time... You don't know how good you have it now kid!".
It also depends on how much the PCs flaunt their cash and how well their armed and armored. You might not want to flaunt wads of 100 dollar bills in a homeless camp. Unless of course you're carrying a machine gun, full body armor and have a couple of friends wearing the same...
Plundering the local megadungeon and dumping all that wealth in the local economy might destroy it, even when taxed into the ground. But what exactly would the PCs buy? What could they buy? Who can buy their expensive stuff? Imagine what the price of gold would do if you suddenly reveal a stash the size of 10x what's currently been mined (not just this year, but ever)? The price would implode. Besides governments wanting you to pay taxes on that gold, possibly economic charges/fines for disrupting the economy, etc. If you didn't reveal that to the world, suddenly the price wouldn't collapse and if you're careful and spend it over decades or even centuries, no harm, no foul... Just better not have a Paladin in the party...
I suspect that many of the transactions between or with adventurers are of the tax evasion kind, you don't buy and/or sell that magic item in a shop, you go through a broker, often the kind of person/creature that doesn't ask a lot of questions, it does ask a hefty fee for it's services, but a lot less then what you would pay in taxes...
Now, you could do many, many cool things with this, but that's way more world building and complexity then most people are comfortable with or are willing to do. And to be honest, if the economy is beliefable enough for the participants, why bother? Even if people were thinking about (all) the implications, things get so complex so quickly that most folks can't keep track of all the many, many moving parts involved. Most just want to tell a story or do an uncomplicated evening of dungeon bashing. Generally the world building, complex economy is more for my own satisfaction as a DM then anything else...