Audiophiles and producers want authentic emulations of specific vintage gear, not generic 'retro' effects. The success of hyper-specific apps like cassette player simulators proves this.
Develop ultra-accurate emulations of iconic devices with custom UI matching the original hardware. Focus on one device per app for authenticity.
Charge premium prices ($10-20) to audio professionals and enthusiasts who value accuracy.
Start with one well-documented device where you can get the DSP right.
Risk is small addressable market for each specific device.