In regards to ‘external synths’, you can either use software synths in the same file format as the built in synths, (for which the only solution is to either build your own as you saw, or to search possibly this forum or the wider internet for links to any that someone has already built), or use Sonic Pi’s MIDI or OSC communication functions to talk to either hardware synths connected to your computer, or separate audio programs on the same computer containing software synths.
On a side note, it has long been my own goal to expand the number and capabilities of built in synths in Sonic Pi - right now I am actually in the process of building a handful of new synths that will be able to create a wide variety of sounds. No firm timeline just yet as to when they will be ready, but my hope is that at least one of them will be ready for the next Sonic Pi release.
In the meantime however, you may find it more useful to experiment with external hardware synths, or talking to other software synths on the same computer, via MIDI or OSC. I personally have never used those functions in any great depth, so someone else here may have a better idea of how to use them than I do!