Hi Ethan,
thanks for this more detailed information (more than my rather ‘frugal’ earlier entry 
) I would like to complement and add that I find the additive approach within Sonic Pi very helpfull to learn the basics of synthesis or for didactic purposes in seminars.
It is probably less suitable for actual sound design. As far as I know decades ago it was the (very short lasting) hope of musicians/composers (such as Stockhausen) and later synthesizer manufacturers to create all sorts of sounds by additive synthesis - which is theoretically possible but practically much to complicated and even more, not feasable given the restraints of hardware power.
PS.: I think it is well invested time to explore the ‘knobs’ which can tweak the available synths within Sonic Pi such as here and here (search this forum e. g. for ‘synth’ and you will find other valuable contributions).
Ah, and here is another thread started by @enkerli about synth design with SuperCollider.