>So if you live in a town with three welders, and those three welders come together and agree that they will not ever weld for below $X/h in the town (which is significantly above market price of nearby, equivalent towns), then that is not price-fixing, because the people that want welding services are suddenly "welder employers" because they are buying a service? I... don't think that's right.

There is a huge difference between this and an Uber driver. All Uber drivers are commodified, there is no distinction between them outside what Uber itself does. Uber's drivers have no control over how they do their job. Uber's drivers have no control over the fares they charge outside accept/refuse.
If the three welders were indistinguishable from each other, and all had the same entity decide vast amounts of how they do their job and for how much, then yes, it would be simply collective bargaining.

Once again, there is massive strawman going on here that you are perpetuating. I'm not saying that it isn't price-fixing. All collective bargaining is price fixing. All of it, without exception. As a society, we allow workers to engage in price fixing because workers have very low individual power and it is in our collective interest for workers to negotiate together. That is the point I'm trying to get across. All square are rectangles, not all rectangles are squares. All collective bargaining is price-fixing, not all price-fixing is collective bargaining.

>The price of rides is determined by the market. Uber arguably has the least say of the three stakeholders – their goal is to merely "make the market". Riders demand low-prices, drivers demand high ones. Uber doesn't really "choose" a price, they just find the happy medium that the market demands.

This is ridiculous on its face. Unless there is a mechanism on Uber for drivers to bid on rides, then fare prices are not fair market prices negotiated between drivers and riders, but instead prices decided between Uber and riders. Uber very certainly chooses a price, and the riders don't. They do not participate in market pricing any more than a factory worker threatening to quit their job contributes to increasing the price of a widget. Most importantly, Uber does not allow the drivers to even know the actual price the riders pay, so this is just ridiculous.

>Uber pays drivers in the same way that Stripe "pays" me when I make a sale of my app – that is to say, not at all. Stripe pays me, but they are clearly not my employer. Uber is collecting payments and then disbursing them, as literally any marketplace does. Does Steam "employ" all the game developers that sell on their platform? Does Etsy employ all the independent makers? Apple's App Store? Twitch? Amazon? Patreon?

If Stripe told you exactly which app to make and how, and did not let you explicitly state which price you would sell the app for, and did not even let you know which price consumers were paying, had a variable cut that is obscure to you, did not let you market your apps individually, and only gave you the option to either make the app or not, then you would be an employee of Stripe, yes. If Steam told you which app to make and how, which price it would give you, didn't tell you how much they would charge customers, and didn't differentiate you from other gamedevs then yes, you would be an employee of Steam. Rinse and repeat.

The core business of Uber, in revenue and valuation, is 100% to provide rides for people. Scooters and bikes are a drop in the bucket, and technically come from an acquisition and thus are not a core business. This is like saying that the core business of Apple isn't to make iPhones, it's connect consumers with engineers and factory workers that make phones. Except the consumers don't choose which engineers make them, how much they are paid, or even really differentiate them before paying at all.

>To compare this to Steam again – the core business of Steam is not "game developers", even though their core business would not exist without game developers. Their business is a marketplace. I actually really like the Steam comparison because it literally ticks all the same boxes you are ticking for Uber: they set prices, accept/disburse payments, and wouldn't exist without 3rd parties and their labor. But nobody is claiming that Steam should start paying health/holidy/etc benefits for all the indie game developers that sell games on their platform.

Steam does not set prices. Steam lets the gamedevs/publishers decide of they price they want. Steam lets you market yourself. Steam does not tell you what game you should make (what route you should take), they do not tell you which programming languages you can use (which cars you can use), and does not rely on contractors for its core business (Steam does not rely on one-person indie game developers for their core business). Steam is incomparable to Uber. The idea of why Uber doesn't let you use any car, by the way, is in order not to damage their brand. Because you, as a driver, represent Uber, and they know it. In a way, your most important asset, Uber's brand, is not owned by you but by Uber - you then generate profit from someone else's capital for a wage - an employee.