Well, I'm concern with the manner in which the artwork is done on the shirts, at least in the initial stages. It would be a lot easier to work out the detail with a company if their business is near. Not to mention, the quality of the shirts being used.
Be it Printful or any of the others, you select the shirts (from the ones they carry) that you want to use. All these places offer you the opportunity to chose higher of lower quality, it is up to you. Everyone has pretty much the same blanks to chose from, as these places have to buy them from the same distributors and manufactures as everyone else, and there are only so many. Best results will be had on 100% ringspun cotton with a high thread count, like Next Level Apparel 3600, Bella+Canvas 3001, American Apparel 2001, Gildan 64000 (solid colors only; the heather G640 colors are 65% polyester).
You'll want to order printed samples from whatever places you are considering. I recommend ordering one at a time on different dates to increase the odds that they will randomly be printed by a different person on a different machine, rather than run as a batch altogether. The biggest issue with PODs (and DTG in general) is that print quality tends to be variable. It is in the nature of the business model that they can't spend time, money, shirts, and ink dialing in perfection on the ONE print of a specific design on a specific garment type that someone bought from you that day. They do have different pretreatment and ink settings for different garment types in general, so they are taking reasonable measures to produce good prints. But it will not be tailored to your art for best results. That is not POD.
You might be able to workout something with a smaller local print company and do the shipping yourself. That sounds more like what you are looking for ... maybe.
The POD/Fulfillment places are large and largely automated. All about pushing volume out the door. Fairly inflexible, in the way of any large organization/machine. You fit to them, not the other way around.
All these places have web sites listing the blanks they carry, pricing, requirements for the art files etc.
I have used Printful and CustomCat (MyLocker if accessed via Printify). I also sell directly on Merch by Amazon, so Amazon prints those shirts. I have seen some really nice prints, and some really disappointing ones, and a lot of "okay" ones. Variable ... 😐