+1 for LFO in Ableton- it’s that moment when you realize Ableton itself is a modular rack. Add in the CV tools and you can have some amazing generative stuff going on inside the DAW itself.
I recently have been playing with the Buchla Easel V from Arturia and their Mellotron V as well, very nice characterful older-type sounds
iPlug2 is also a cool C++ framework (and free) for targetting multiple plugin formats. Probably gonna be more frustrating than JUCE because it’s in “pre-release” and there’s much less documentation, but hey, no splash screen! It’s predecessor, wdl-ol, is pretty similar and has more documentation but they’ve deprecated it. The original iPlug was actually made by the people behind Reaper!
In terms of like general advice, the musicdsp website has a lot of cool algorithms that can be helpful. “The Computer Music Tutorial” by Curtis Roads helped me a lot in undergrad but a lot of it is pretty complex/dated and more high-level/conceptual. It’s worth brushing up on or learning advanced math, calculus/trig/algebraic things come up a lot in dsp.