On the assumption that I know C++/Qt, and python/pyside already, and have a geospatial and open source background -- but have never dug into the internals of QGIS in any capacity?
Thinking of writing import plugins for geophysical data (things like ground penetrating radars and such).
I'm realizing my knowledge of how to do this is really dated. I got into QGIS plugins 15 years ago, and at that time there were some links on the QGIS website to plugin tutorials across the web. I'm not finding that now.
I do know I started with the QGIS documentation, and it looks like these days they point you to the PyQGIS Developer Cookbook as the starting point. It is also probably worth wandering around the dedicated QGIS plugin website. I strongly suspect you've already done both of these, though.
I do remember that the learning curve was steeper than I expected. If you go the route of a Python plugin like I did, before it is said and done, you'll likely need to be comfortable with at least some Qt, PyQt, the QGIS Python API, and details about the tools to manipulate the data you're working with. In my case that was PostGIS and pyscopg2, the Python PostgreSQL library.
I'd strongly recommend you start very simple. Manipulate a few things in the Python console. Work your way up to a bare bones plugin that provides a single menu option. Then start building up from there.
I'll be very curious to see if others have links to tutorials they've tried recently. There's the usual route of checking Google and YouTube, but I cannot tell you what is reliable there and what isn't.