If no one ever spends a dime on the service, you've always got one happy customer (yourself). Don't think about customers, build it for yourself and solve your own use case. If it's a good product, customers will find you/it. As soon as you get into tailoring it towards the needs of paying customers you've lost your original battle of creating something you enjoy for your own needs.
To be honest, I am way too invested already to not finish this thing, despite whatever response I get from posts like this. Sometimes I do get curious though. A surprising number of people I've asked in person simply are not interested in saving things from the internet. I guess if all you are into is watching Snapchat and Instagram all day, why would you?
This makes me think I need to tap into the larger business-type market and has me looking into what it would take to implement a permissions system for teams/multi-user editing of notes/etc.
No such thing. That's the sunk cost fallacy. But you probably already know that and are expressing your joy in the project instead of literal investment.
Failing faster and more often is a good strategy. There's a great Freakonomics podcast titled "Failure Is Your Friend" with a bit more context and justification.
However, I would never rent note taking software. Notes are much too valuable to just have an app/website shut off one day. I want to own the app so that if my system state does not change, I can continue to use it indefinitely. My price range would be somewhere around $50-75 every 18-24 months. I think of utility software like a drill from Home Depot. I expect software to run for 18-24 months and if it breaks after that period, it is up to me to revert to a system the drill works on or buy a new drill. 18-24 months feels like a reasonable timeframe to me. Essentially, it is a tool with a two year warranty.
In addition to valuing ownership of the software, I also value ownership of the data. I (or a different 3rd party I am comfortable with) would absolutely have to be in control of the sync process. Ideally, it would be a folder we can stick in any old cloud service. I value both the data that is in those notes and my privacy too much to trust any random sync provider. I would also prefer either a standard format or a well documented storage format (like Quiver has done) and a way to get my notes out of the app into a reasonable format.
Apologies if this comes across as an attack on your idea. I just wanted to voice my opinion as I have dropped 3-5 pieces of software in the last 18 months due to a combination of moving to SaaS and/or forcing me to use/pay for their sync services.
Having said that, I have paid for writing/note taking tools in the past. I bought Ulysses which is a Markdown note taking/writing app. It's available for Mac and iOS. That was a one-off payment of around $30 for the Mac app. I loved it, used it for years, but moved away when they introduced a subscription model. I do still occasionally use my paid for app though.
With regards your $5/$10 a month - personally I would not pay it. I know that because I was paying around $7 a month for a private GitHub repo and that cost annoyed me. It doesn't sound a lot, but over years it does add up.
So, I guess, are the new features you are going to build into your app worth 5/10 bucks a month?
For this reason, having used many different systems over the years, I won't ever again use an online notes-island not under my control. I think this is why when this comes up, so many people respond with "markdown!", "org-mode!" etc. These can be invested in without the stranding risk.
The issue is that yours won't be just right either. Except maybe for you, and hopefully enough others to keep it afloat.
For a while I used OneNote heavily even though it is by no means optimized for writing technical notes (formatting source code is much harder than it should be; stupid Microsoft killed OneNote by obnoxious marketing methods such as stuffing multiple links for it on the task bar, defaulting by sending anything you print into OneNote, etc. They never thought that devs are an important market.)
For me the good thing about OneNote is that the full-text search actually works. Really, I'm not kidding. I have no trouble finding things in OneNote and that is worth a lot.
For some reason everybody wants to write a new notetaking application, but the problem is the proliferation of applications. One team is into Slack, another uses Discord, so I have about 10 electron apps that are all almost exactly the same size.
The answer is something that can go into all of these spaces and make things findable first, and portable second.
Nobody seems to want to do it, but everybody seems to want to learn Electron.