Geekage I looked at other service providers when I first started, and as flawed as Shapeways is, it was the best option at the time. Now with seven years at Shapeways, I am pretty much tied to them. It is even to the point where several times a day I have to tell people that message me that I am not a Shapeways employee.
With respect to making the models available, there are options. I would totally skip Thingiverse, that place is a mess. Right now I think MyMiniFactory might be the better of the rest, but I can't say for sure. You can put the models up for self-printing, but your concern for copycats is real and likely the first place you will find your models for sale is eBay. What I did in the past was to post early versions of my models in 3D file repositories. Usually it was just serviceable versions, very blocky, maybe missing some refinements, maybe in a single size. An example, I remodeled the Garmin quarter-turn mount to make it what Garmin should have in the first place, on the repositories I used a model of the original quarter-turn instead of my remodeled eighth-turn mount. And, my eighth-turn model has been copied by someone and that version is up on Thingiverse somewhere (no idea if it works as well as my version and I don't care to find out).
If you allow others to print it themselves, please note you have no control over the skill level of the person printing it. I don't think you want to be in that position if you see this becoming more of a business than a hobby.
If we stick with Shapeways for example, they have a few options for printing that they do not offer in their marketplace. MJF PA12 w/glass beads is a much more rigid offering than the normal MJF prints.
One more tip, if you are going to prototype a lot of models, buy yourself a resin printer and become very proficient at using it. Especially with the new more flexible and robust resins, and engineering resins available. That and a bit of tweaking to the printer build volume numbers, you can get precise size specific models printed, and you can print a plate full of prototypes at once. Youtube has a rash of videos showing some impressive results with cheap small MSLA printers like the Anycubic Mono, Elegoo Mars 2 Pro Mono, Phrozen Sonic Mini 4K, among others. Larger format if you need bigger parts might be the Anycubic Mono X, Elegoo Saturn, Epax E10 Mono, Phrozen Mighty 4K. The parts that come off those printers might not suitable for extended outdoor use as they continue to cure in sunlight and tend to yellow or even shrink a bit over time. It does work very well for prototyping without waiting on your service provider to print, ship, repeat. Yes, resin printers are messy, but your return is that you will be able to print parts that look finished and don't require any post work to be usable. The detail you can print with rivals and even supasses the stuff you can get from Shapeways or any of the other print providers. I have totally given up on FDM printing for printing test parts.
The parts thing is real, even screws that are readily available at most home centers and hardware stores as well as online, I know I lose sales from not being able to supply them with the models I offer. I can't tell you how many sales, but I am sure there are some. A lot of people want a complete solution, nothing left to acquire, all in one package.