How should I choose optimal in-app events to bid against at the start of a Google UAC campaign?

I would advise to run two campaigns in parrallel at the beginning to see which one performs the best. Last time I did that with my own campaigns, I quickly realised that, even though the conversion rate for my campaigns was higher for my campaign that was optimizing towards the event (sign up), the campaign that was optimising for the install actually had a better Cost per Sign up, as the orginial cost per install was much lower. After a few weeks of trial, seeing that my CPI campaign was still performing better, I then paused the Cost per event one to avoid the two campaigns to compete with each other and get more volume.

So I would advise to run two campaigns: one optimising towards the installs, and the second one towards your most important event, and keep the best performing campaign after 3 weeks of trial

Hope it helps!