This one is basically my daily driver for similar tasks: https://app.heynote.com/ (full app at https://heynote.com/). And it seems to work almost out-of-box for your first two examples, once you switch the buffer from "Plain-text" into "Math" mode.
It also supports switching between different buffers and some kind of local storage.
Umm, yes? Not sure about the rest of Europe, but here in Czechia most of merchants have a direct payment method based on QR code.
Basically: 1) you select your bank at the checkout; 2) you're redirected to the bank's payment page with a QR code; 3) you scan the code from your banking app and confirm it; 4) the bank redirects back the merchants page with the payment status confirmed/declined. No payment processor involved and the money goes straight from your account to merchant's account.
I would say that the problem really is there. Dealing with native dependencies and addons was almost always a pain as the article describes (and not just from developer perspective), so anything that helps there is really appreciated.
Not sure what you mean by the right tools in this context.
Code testing is a big one for me. I'm currently using in-memory sqlite for tests and I'm often running into differences between sqlite and postgres (default values, JSON handling, etc). This could allow me to use the real thing without running a full Docker instance.
It also supports switching between different buffers and some kind of local storage.