I think the piano keys will be the only truly hard part.
The electronics and code is a 1 day to 1 week kind of thing to get started.
You will want a custom PCB, but if this is at all boutique-ish, you can probably afford the $5 for an off the shelf ESP32 or RP2040 module with USB Bluetooth built in.
I would guess the PCB fab costs will only be $30-$40 even in single device quantity for a decent sized board, less for a small one, it's insanely cheap now.
If there's any hard parts in the code it will be in whatever fancy customization settings stuff you need.
If there is no fancy UI and it's just a fixed mapping, then software should be trivial.
If this is low volume enough to use 3D printing, I wouldn't completely write off being able to make the keyboard from scratch.
I suspect but can't say for sure, that starting with only stuff a SW engineer would have, and assuming unlimited free time, and already having all the skills, a prototype would cost well under $1000, maybe even $500, with the main expensive part being a 3D printer.
If I were doing one, I would probably start by prototyping a single key in a few variants, along with the associated electronics on a breadboard.
I think I'd estimate about $160 of work and $40 of parts(Since I already have the tools) to try a few ideas over a day or so, just for some preliminary research, but a real product engineering firm could be "If you need to ask you can't afford it" kind of money....
In volumes too large to just 3D print, there's a lot more to think about, you'll want someone with molding experience, since the design rules are rather different, and the cost of failure is way higher when you're actually having to make molds.
Oh, and whatever you do: don't use a MicroUSB connector!! Please! I have seen way too much audio stuff with a delicate flimsy MicroUSB on it! USB-C or printer USB is worth it!
Also don't put too much weight on online electronics advice: A lot of it is geared towards people who want to do more analog stuff. The skills may have a lot of overlap, but the gear is a bit simpler for this kind of relatively basic digital work.
Just ignore any of those mile long essential parts and tools lists, you probably only need a quarter of that stuff.
I have no idea about the marketing side or the HR side of this though. It seems like boutique musical gear is kind of a big deal though, so I would assume it doesn't have to be ultra low cost?
I'm happy to answer any further questions, most of my background is working on ultralow budget projects for companies without much hardware experience.
Business and marketing and all that is hard, making small quantities is something you could probably do in a month or two if you were really motivated.
It really is amazing the difference in difficulty between making one or two of something and actually going to production!
Also I should probably ask the obvious question: does this need pressure sensitivity, and does it need aftertouch? If not, you can probably find mechanical keyboards keyswitches that work.
If you need sensitivity but not aftertouch, you can probably just use pairs of keys at different positions, and detect the time interval between them.
If you need both, I think I'd try Velostat rubber as the sensing element.
If you need it to be weighted and feel like a real piano, it's probably going to be way harder.