Something I learned was when designing real products, as opposed to tinkering about, you need to comsider the form factor and envelope much earlier, as that will frequently define your board dimensions.
Trying to jury rig a random board into an enclosure it wasn't designed for will almost certainly be messy.
You can of course have custom enclosures made, but tooling is invariably going to be very expensive. Usually for low to medium low volume, it's common to use an off the shelf enclosure, so you pretty much have to design your board around the chosen enclosure's envelope. In many ways that latter method makes things easier because many previously flexible parameters are effectively forced on you, so you spend less time procrastinating.
3D printers, unless you have access to a seriously expensive one, are OK for prototypes and rough fit checks, but are not for commercial products. Sure you can use the parameters to have your enclosure made properly, but unless it's high volume or high value, I'd use an off the shelf enclosure, or else you'll find your enclosure costing the majority of your BOM. Depending on your market though, the target audience might demand a custom enclosure, but for low volume I'd avoid it. If I had industrial design skill sets, I might have a different conclusion though!
One thing I would say is that there is a world of difference between having something working on the bench and having it manufactured. If you're "not... very motivated to investigate" then my first thoughts are maybe you should stop right now.
Selling on the web is easy, essentially it's a Wordpress website and a Paypal button. Much as there is a vociferous hatred of Paypal, and much of it is justified, its ubiquity means it's the most sensible way to start receiving money. Setting up an eCommerce website and credit card payment provider interface, especially for international use, is a very significant undertaking, which is why I recommend a simple Paypal button on a web page.
Dealing with Paypal though is a right royal pain in the ass when you first start bringing in significant funds, particularly in that they withhold funds sometimes for a couple of weeks even though you've delivered product. Once you're established, payment time becomes immediate, but it takes a month or two for that to happen. Even if you have an established personal account, that counts for nothing as far as Paypal are concerned. There are several other gotchas with Paypal, such as don't access your account from abroad, as that will trigger your account back into a two week payment delay mode for example. (I access via my home VPN to avoid this).
You might also want to consider whether you want to find a retailer or distributor, but whatever you do don't work to silly margins unless you think what you're doing is a charity. If it costs you, say, $35 COGS/unit in volume you should be looking at at least $100 retail. The retailer will be paying you around $70-$75 per unit. You have to keep a roof over your head. You are the one taking the risk by investing in the product. The product needs to pay its way, and it is right to expect a return on your investment. Don't be afraid of making a profit. It is a lot easier to reduce price than it is to raise it. If you have a unique product that people want then people will pay.
For 20 units, I assume you'll make those yourself. My first product, I made the first hundred or so myself, which had about 120 parts per board, all smd. It was not a career I wanted to get into, hand placing parts. You need a board assembler and the best way to find one is on personal recommendation. Do not be tempted to go to China just because it's cheaper, because I guarantee you that for small/medium volumes in the end it won't be. There is a different culture, you think you have a deal, they will use all the parts as specified on your BOM, and then they go and substitute crappy parts. It happened to me, it happened to Raspbery Pi, and it'll happen to you!
I am not sure what the "market tests" are for? Put your gadget up for sale, see if it sells.
Regarding packaging, if it's a gadget, frankly I'd find a reasonable existing plain box that can be had in bulk for pennies and use that. Print some stickers out and but them on the box.
Lastly there is the thorny issue of certifications and type approval, but it very much depends on your particular device and the markets you intend to sell into as to what that will entail.
It's not that it's not fun, but a lot of it can be challenging and frustrating, particularly when manufacturers let you down or parts become unavailable. The best bit was being able to give up my day job.