Yeah, I'm with you on starting with Amazon initially. I didn't mention that the price of the products isn't always that high, but the majority of the "well known" brands (eg. Nike, Samsung, Asus, etc.) in my niche are in that price range I mentioned (they make around 95% of the available models in my niche). The most popular and always in stock options on Amazon are in the $200-800 range and those are what I like to call "Amazon no-name brands". I'm sure you saw dozens of these. They exist only inside of Amazon and don't sell anywhere else. They are usually the budget alternative and are almost always in stock and they have tons of reviews and orders.
I would definitely agree that there aren't impulse buys in this niche. People usually buy this product and then potentially stick with it for decades. So yeah, the longer cookie makes sense. I just did some digging and most of the affiliate programs in my niche offer a 14-day cookie which isn't ideal but definitely better than Amazon.
The PPC testing idea sounds great, I'll actually most likely give that a try when I have time. Thank you for that one! I would definitely like to go with Amazon for this project because the difference in commission rate isn't that drastic but if people end up buying the stuff after a week or two then it's all in vain.