Where are Commerce 2 hooks (or Events) in Commerce 2
as you've discovered, you can look in any module's src/Event/*Events.php file to find a list of their events. Using the order paid event is one way to react once you know that payment has been collected, but note that in scenarios where an order is placed with only a payment authorization (and not a capture), that event would not have been fired yet.
That may be fine in your case - if not, I'd recommend the commerce_order.place.pre_transition or commerce_order.place.post_transition events that are triggered when the order is placed. That event is coming from the state machine module and correlates to order workflow transitions. You can read more about it in the module's readme.
As for form alters, those exist in D8 as they did in D7.