DPF VS essentia

DPF, short name for Distrho Plugin Framework, is a framework for building audio plugins in C++, compared to JUCE is smaller and more "raw", but there's no commercial licensing or limitation, also it supports open formats like LADSPA and LV2. It has support for Linux and Windows, here I will assume you're using Linux, so some commands may differ in Windows.

For crossplatform plugin building - consider checking out https://github.com/DISTRHO/DPF

Re 1: Why not create a DPF wrapper for this and have DPF create the ladspa/dssi/vst2/vst3/lv2 for you? -> https://github.com/DISTRHO/DPF

Nice to see you are interested in making audio plugins. Especially with LV2 support. OK, you can do LV2 plugin programming from scratch (like I did). The LV2 book https://lv2plug.in/book/ is a good starting point. But I would recommend the use of a plugin framework like DPF https://github.com/DISTRHO/DPF instead.

I use DPF as a plugin framework so any wayland stuff should come to that. I have no clue how to implement such features. I'm not a real programmer. I only act as one on the internet.

Alternatively you could look at using DPF to make the plugin: https://github.com/DISTRHO/DPF

There are free (as far as licensing goes) alternatives such as DPF, but you will be doing a lot more of your own legwork with this system as not as much is provided for you.

I'd also recommend Essentia - example of rhythm detection here. It's an all-round great library for audio analysis/synthesis.