Pretty much everything you've said agrees with my past experience.
I would also add one inference I've made over the last several months that may (or may not) be true... It seems like new ads tend to get a boost when you first launch them, like a trial period I guess, but that doesn't hold true if you keep launching new ads for the same book repeatedly during a short period of time. There seems to be a diminishing returns thing going on there.
And if you let an ad rest, or stop advertising a particular book for several weeks, when you start advertising again it gets another chance to be brand new.
I've also found that it's a good general practice is to try to keep Amazon's best interests in mind with your ad. Basically, they like ads that make a profit for them. So if your ad is making tons of money for you, but you're using tiny bids and getting a bad CTR, you're probably taking up valuable carousel space, and Amazon will gradually phase your ad out. But that doesn't mean small bids can't work, you just need a good CTR. Sales conversions might even factor into it as well, but it's hard to be sure.
In a weird way, keeping an ad alive and thriving is almost like caring for a plant, and it seems like over time you're fighting against a tide that slowly pushes old ads down in favor of new ads, very similar to the way new release books gradually push older books further down in the charts.
For myself, I haven't been able to achieve anything truly consistent with AMS ads. I've had some ads that did great for a long stretch, others where I lost money. Right now I'm not running any at all, but overall I've made a small profit with my Amazon ads, and I'm getting better with them.
Sometimes I feel like the rules are constantly changing. I think Amazon are still tweaking the baseline algorithm for their ads, so trying to work out a reliable strategy is equivalent to shooting at a moving target.
