Question: When you say "best/fastest signal" and "prove that your product is something people are willing to pay for", are you speaking of proving to myself, like a bootstrapped small business? Or of proving to prospective investors?
Regarding the paid&free model, just thinking aloud... I suspect that the browser vendors are going to push me to use their extension store, so maybe I have to put the paid features as data/behavior on a server that the extension talks to. Or to send a paying customer an unlock code that flips a trivially-crackable switch in the free extension, but which at least helps keep honest users honest.
You have the advantage of being able to start and potentially get something into the market without needing to take investor money and promise larger than life returns on the money you take. Use this time wisely to make an honest judgement of what you think this business could turn into, if it's just something that pays your rent then let it be that. If you grow like crazy and need investment money, only then pursue it.
Dealing with investors is a really big headache, so only accept money from them if you really think it will make a difference in being able to grow the business.
Yes you will almost certainly have to use one or many extension stores, you will also need to setup a secure way to handle subscriptions and give elevated access to users that pay.
I write code in C# or C++ (but also Python, pipelines, scripts, JS, etc.) and it needs to be somewhat efficient. We all follow the same rules (62304 especially), we must write unit tests, and make sure that my features are properly integrated at all steps of the development up to the release, and even after when you must validate it with the authorities, when you have bugs, etc. If you're in a small company, everyone can be involved in all the processes and it's fun because you go much further than mere development (like preparing reports for various agencies all over the world, or helping PhDs integrate their code in the application).
We have commit hooks to check and format code, pipelines must be green. You cannot cheat because someone will find out. And you can’t pretend that your code worked once on your computer because the test team will refuse your code if it breaks anything. It’s more rigorous at all steps of the development.
Last but not least we have specifications for everything because it’s the law. Overall it’s what software engineering should have been all the time. It feels like working at NASA even if it's only a stupid desktop tool or application.
Of course everything is not perfect, you can stumble on assholes like every other company, but it's not everywhere. I’m happy to go to work every day, I may have saved a life or two with my code, and that's a good feeling.
My experience comes from having worked with the biggest assholes on the planet at different companies. To answer your question, I would say that an interesting job is rigorous, peaceful, and has some kind of meaning.
Edit: as another guy said, I too settled for lesser wage to work for a company that would not destroy my soul and spirit. That's important too.