Monetarily? No, not at all. Repairing computers hasn't really been profitable for years. If you want to constantly play with new computers and not really lose any money on it then yes you can probably not lose money on it, but don't expect to make money.

Perhaps if I get a laptop that’s compatible with Libreboot, such as the Lenovo ThinkPad T480, I could also flash Libreboot onto it.

No normal person wants this. That's an extremely niche demographic. And the demographic that wants this, is almost entirely going to be the same demographic that would want to flash it themselves.

In order to be profitable you will need to either pull the laptops out of the garbage or get a really good deal. Most of the time, the profit margin is too thin to be viable. I used to get laptops for $10 to $20 at Goodwill, but they would need $50 to $100 worth of parts and only sell for $100 to $200. After labor, parts, etc. You would be lucky to break even. I would spend more time looking for some good repairable laptops at a decent price, then actually selling them. Maybe if I had a store and people brought me laptops to buy, it could be profitable.. But its extremely thin margins.

No one is going to want to but some weird ass operating system

I buy almost exclusivly second hand electronics. The existing market for refurbished computers is large, already.

The ones I buy seem to be written of hardware from companies, that required very little work to refurbish.

I'm simply guessing, but I expect that there's not much profit to be made, if you take into account your time worked.

How many people do you know who have bought something like this?

That's where you start, see what the existing market is doing, and if there is a gap.

Sounds like a good way to afford your hobby, but nothing you can live off when having to compete with big players like refurbed that can outsource the work to low wage countries.

It's doable but it'll take a lot of learning and experience to actually make money. There's always plenty of stuff available that is in working order for pretty low prices so competing with that will be tough. Also there are lots of established players in that field, so it'll be hard to compete.