No courses on the 3D printing side really — I think I did go through one of the Prusa ones after getting enough free "meters" on Printables but I don't remember it telling me much I hadn't already learned. There is a blog post that has been posted here before that really covers almost everything important about design for 3D printing:
https://blog.rahix.de/design-for-3d-printing/
I really just ADHD'd the hell out of it, I suspect [0], and absorbed everything I read. I was in financial difficulty and things were expensive so it took me a couple of years to get me from "I'd like a 3D printer" to "this 3D printer is affordable but viable and even if I never learn design there are plenty of tools I can make with it that will save me money".
In that time I read everything I could about what I'd need to learn, convinced myself that I was not so clumsy and inept I couldn't maintain a printer. These days printers don't need so much mechanical knowledge to get started.
On the CAD side of things, I learned a bit of OpenSCAD, found it basically helpful to make one simple thing but also frustrating and disappointing, joined really useful non-public Facebook groups where people were working on similar things, decided to get properly into FreeCAD, and dug in with the Mango Jelly Solutions videos on Youtube (which actually are now organised into a course structure, but weren't really then).
The thing that motivated me mostly was having simple real things I wanted to make for a project I was working on (though my brain being what it is, I still haven't got round to that exact project...)
If you have a need for a thing you would like, and you're able to break it down into simpler projects, particularly if they are things you might find useful along the way, it's not very difficult to find the motivation to learn these two things.
The positive feedback loop is so strong, and 3D printing is such a concrete way to learn CAD and design because you get to hold your design so quickly: I designed this thing in CAD, I printed this thing, wow it works but I could improve this, I need to learn this new thing in CAD, I printed it, it works but… etc.
Pretty soon you find yourself staring at some real world object on your desk and modelling it in CAD for fun.
The really interesting thing is when you begin to understand that the design of things is fundamentally influenced by the tooling used to make them. When you grasp how 3D printing and injection moulding differ, for example, and start designing your own items with respect to the strengths and weaknesses of 3D printing, rather than just to look like an existing plastic part which was moulded, then you're really getting there.
[0] hilariously I am still not formally diagnosed. Though I'm pretty sure I could be diagnosed just based on these two comments.