I used the JUCE framework, which is free for personal/GPL usage (or you can pay for a closed source license) and gives you cross platform. The examples are pretty good for getting the basics, there is also The Audio Programmer YouTube channel that has a lot of content (the only bad thing I would say about it is that some of the early videos sometimes use bad concepts as it seems this was a sort of a documentary series for learning plugin programming for the channel owner). The JUCE community forum is really friendly and helpful (and there is also a discord channel for TAP). Otherwise search engines were my main source to learn (mostly to refresh my C++ knowledge), and I managed to land myself a day job programming a plugin that helped a lot just by looking at the existing code, but that obviously a very rare occurrence!
Slowly putting everything on kvr and have been very slack with the YouTube videos. I think mainly have been blessed by having our record label with same brand be the main driver for sales, and I've been asking well known artists in the psytrance scene to help with promotion (being my friends I give them freebies and ask without obligation to post something about them on their social media).
Probably don't have many sales outside that scene to be honest, so I'm hoping with time that will change.
Can you link to any resources you used in learning to build them?