Onyx does not natively support TapTempo for FX speed unfortunately.
However, if you use ShowCockpit I’ve written a script that does exactly that (in this script I’m using a playback button for the TapTempo effect, but it could be configured in ShowCockpit to execute the script from any kind of MIDI-button, Keyboard button, Stream Deck button e.t.c):
(it changes the playback fader value on each tap - which means that you can easily override it by just moving the fader)