I am looking at making a start using FBA as a side hustle for some extra income
The model is getting crazy saturated. If you treat it like a side hustle, you likely will get caught in the ongoing consolidation as high rollers are outspending smaller, inexperienced players (massive giveaways, massive PPC bids, selling at cost). If you treat it as building a true and unique brand and business, you will have an excellent gateway channel (the best really) into the world of ecomm.
Forget ASM, if you search for it all the same info is out there for free (particularly in podcast form). The easy days of this where you hit a home run out of the park with a generic product and a 'brand' sticker on the box are gone. I'm not suggesting not to do this, I'm simply commenting on the current environment.