I might have an effect knob that I want to automatically vary depending on how something sounds, most likely by the modulator sound's overall loudness or real-time peaking frequency, like an LFO attack or phaser or ring modulator or w/e. Is there any plugin that can accomplish that?
For loudness, lots of plugins have envelope followers. An envelope follower converts loudness into a control signal that you can route to a knob.
Some plugins have pitch detectors that convert pitch into control signals. For example, most of MeldaProduction's effects have that feature. See their "Pitch Mode" modulators.
Envelope followers are pretty common. Pitch detectors are less common.