The better way to learn FBA is to sell third party one way or another. That way you don't have to learn Amazon and learn starting a brand at the same time. It is easier to learn Amazon independent of all the difficult issues involved in both building up a brand and building up an ASIN. It is so much easier to make money with pocket change startup costs by just selling popular products or popular brands that are not locked down.
JungleScout and similar tools are also extremely misleading to new sellers particularly now in a crowded marketplace.
So many things can go wrong with a shipment also.
You can easily start a profitable but very small FBA business with nothing but an introductory 0% rate business credit card that you pay off every month to avoid winding up in a nasty situation. $5,000 or less would be ample and the only business plan you need is "buy a few units of something, if it sells well, buy some more units of it, and if it stops selling or you get into trouble selling it go sell out of it and find something else."
Starting a brand from nothing is really expensive and challenging particularly if there is nothing that fundamentally unique about the product.