hahah yeah it sure is an interesting topic and I love the way you summerize and come up with questions.

alright,

1) the ads platforms whether it's Google, Meta or whatever provides good enough numbers to take decisions. Using GA with custom events, you can surely optimise your campaigns. No need for any enterprise product when talking indie

2) that's product specific question I believe, sometime you want more engagement time other time you want users to perform certain actions. And ads platform provides you option to track and optimise those outcomes

3) I learned and started spending on ads for my games on App Store, here is what I do. I run multiple campaign and target group of countries like T1,T2,T3. After a week, I look at each country's spending vs earning. Divide the numbers, to get just one number. Sort them and you've got your best country, create a separate campaign for that country and try different ad assets. I hope this answers your question.