You missed the widgets: chatbots, mini-apps, browser extensions, add-ons, etc.

Today lots of major successful products have an ecosystem for extensions/add-ons.

You can find a missing feature in such a product and implement a paid extension for it.

Pros:

- easier to build

- can be promoted via platform stores/markets/directories

Cons:

- limited

- 3rd-party platform risk

It's best to use a widget to validate the idea, and then rewrite it either as a mobile or web app (when it makes sense, or possible at all).