@Nikos I’ve only been using Sonic Pi for a couple of months now but FWIW I posted up my attempt at creating (possibly over-engineering!) a way of combining and detuning multiple oscillators here.
Unfortunately, being fairly new to electronic music production in general, I was unaware but quickly realised just how complex sound design actually is (don’t even get me started on phasing issues 
). So, for me, I’ve instead headed down the path of using Sonic Pi to send Midi to a Synth VSTi where all of the smarts around dealing with multiple voices and phasing are built in.
But, having said that, I would sincerely love to hear from some of the veterans on their thoughts on the past, present and future of rich sound design in Sonic Pi 