There's a whole bunch of apps for invoicing, but if you try them, you'll see that they are excessively complicated. Probably because they want to cover all bases of all use cases. Meaning they aren't great for any use case. Like you say.

The invoicing app in particular I was referring to is Cakedesk. Made by a solo developer who sells it for a fair price. Easy to use and has all the necessary functions. Probably the name and the icon is holding him back, though. As far as I understand, the app is mostly a database and an Electron/Chromium front-end, all local on your computer. Probably very simple and uninteresting for a programmer, but extremely interesting for customers who have a problem to solve.