Coming back to this as an update, but IMHO a very smoky mezcal works best here. For the first time I used Montelobos and the second time I used Lopez Real. They are both great, but the Montelobos has a very smoky flavor that balances very nicely with the angostura bitters, while the Lopez Real felt a little one dimensional.
I tried again with the Trader Joe's mezcal (tolerable for $22USD), upped to 55ml, and 25ml lime, which balanced the Ango nicely!