A few general thoughts that might or might not useful depending on your situation:
Not sure what your price point is, but imo, don't sell anything that doesn't have a high enough price point that you basically don't care about the fees (unless you want to get into some sort of mass production). Otherwise, you'll just be buying yourself a shitty part time job. FAR, FAR better to sell one thing per week at $500 than 50 at $10. Also, if you have a high enough price point, you can afford to spend a bit on ads.
Nothing wrong with selling on Etsy, but if you don't mind pre-building your product, sell on Amazon and have Amazon do the fulfillment for you. You can also have Amazon do the fulfillment on things you sell yourself or on other sites. This way, you can just send big shipments of stuff to Amazon all at once so you don't need to deal with it yourself, one at a time. Note that this works best with relatively small, expensive items. For bigger/heavier things, oversize charges can get prohibitive.
Also, this might sound obvious or impossible depending on what you want to sell, but try to make your product something that people are actively searching for with no great solution. It sounds like you may have something in that ballpark, or you would have bought it instead of trying to make it yourself. That will not only make it easier to sell on the ecom sites, but it will make it easier to funnel people to your own site so you can sell directly.
