Good Luck!
Besides that, always follow the AMZ-FBA Guidelines, like not putting an Insert Card that redirects to your shop (it doesn't matter if there is a discount or anything)—you get a permanent ban.
I saw huge sellers - Millionaires who were BIG in retail but had no knowledge online; they made that kind of Mistake, more mistakes, taxes, etc. Bam - Down.
Im operating in this field a long time, always Product Hunting, but since 3 Niches already convinced me - I regged them as Trademark, and bought Domains, so all the Admin Staff before even making 1EURO.
Well, that's Germany.
Although I was not in the E-Commerce field, I learned a lot as an employee about managing complex projects, forecasting, and other similar tasks. But those are things Amazon FBA tools do for you.
You could use several excellent tools - expensive but well worth it.
I made the same mistakes, learned, stood up, fell again, stood up, and now it's going (thank God) pretty well, so I left my main Job and focused entirely on E-Commerce.
However, since I have this drive, I still proceed with other things, like creating content for YouTube, managing influencers, and building an SMM panel.
Anyways: For E-Commerce, at least in my Niche, where the product range is (on AMZ) around 30-40Euro / Dollar, you are unfortunately even "small" when you order a Container worth 60-70k and still have to wait, like a dmbass, because there are huge Sellers out there.
Cutting Delivery Costs, Designer for the Pattern of the Item, Licenses like GTIN (in Europe), this Barcode, and many more little things kill your Margin.
So keep an eye on every little Penny you spend.
You can make some little tweaks to outperform your closest competitors, a kind of "grey hat," but it—at least for two of my brands—is not that easy to outperform. Almost everything, except taxes, gets reinvested.
I have never worked with PCP, etc. I am just doing regular Sales with good SEO, but I will consider this Block as well because the next Delivery is coming in one Month.
If you are not selling small Items (because Big items are obviously expensive for Warehouse storage) and have the capacity at home or rent a small Warehouse, then make a Calculation and include it in your Forecasts.
Do not forget the "indirect" (sorry, I'm not a native speaker) costs, like human capital: The time you invest, while you could do other things, is also a key factor for me. Not that I'm calling myself a "Hustler," but in my previous jobs, working up to 80 hours a week, it was hard to get rid of this attitude. It's 05:37 a.m., and I'm still working on other things.
So keep this also in mind: Do not work that long, prioritize things properly, and most importantly, if the train is already running, you are probably too late, and sunk costs will hurt you.
I made those mistakes, as I said, with Dropshipping as well.
Pick the Products with a clear mind, don't fall in love with them, and always keep the price (your own budget planning) in mind—and you are good to go!
Etsy is also something I want to step into. But there are too many things to consider, even for me, who has the practical knowledge (low-mid)—my Partners (Uncle & Dad—experts in Retail)—I'm more or less the online counterpart.
And the advice I got from them was the best.
Negotiate, negotiate, negotiate—even if they say the last price, play hard. Build a good relationship with the Supplier, and you will succeed.
What i need to learn now is propper targeting for my products.
If you have any questions regarding Product hunting, trends, etc., I could help without selling anything or something like that—we can talk about that in chat here.
Besides learning things on BHW regarding SEO, I'm thinking of some Business Ideas for Etsy, preferably digital goods, which are also oversaturated but still trial and error.
So the beginning will be hard if you decide - I know as of now you don't want to, but in case you have your trademark and want to product products on your behalf, don't forget to Apply for Amazon Registry Brands - as I already said previously

It's a pretty weird and complicated process that you have to follow.
But the advantages are worth it. Just the ABC layout you can implement on your Selling Page makes it pretty attractive!
I am super excited about your Journey following this Thread.
It would also be interesting if Dropshipping AMZ to ebay still is worth it.
To answer the eBay question in general (someone asked): Yes, at least in Europe, there's still one big marketplace to play.
Good luck!
Kind regards,
zotix