I think you are overthinking the definition of an investment.
You are basically saying that in order for it to be an investment, you must perform any job that pays in fiat then use that fiat to purchase bitcoin for it to be an investment. But you can cut the middle man and skip some steps by performing a job that pays in bitcoin and simplify the process.
And it is a great investment too because such jobs could be performed as secondary jobs where you see that as an additional revenue too. For example I remember many years ago I helped someone with something very simple (drawing charts and some analysis using Microsoft Excel) and got paid something like 50 bucks. Those coins (which I still hold) are now worth more than a grand. Is that not an investment?
Investment means you have something by some ways and hold it for a long time with strong believe of its value growth. Yes, value growth, not price growth as price comes after the value, and if there is no value, there will be zero price. It is the same, if there is no value growth, there will be no sustainable price growth.
Investors are people who focus on fundamentals of Bitcoin as they know that these things make Bitcoin value, and from good value, Bitcoin will be traded and changed hands on exchanges with good price and big trading volume as demand is huge. As principle of supply and demand, price will be positively lifted upwards with time and if we look at Bitcoin price in yearly lowest price, the trend is bullish.
Investment can be from spending money for buying bitcoin or work for bitcoin directly in salary, or do anything that can have payments even in shitcoins but then you determinant convert shitcoins to bitcoins for long term holding. All of these practice are parts of Bitcoin investment and an investor can have all of these things or just one of them.