Infill has massively diminishing returns. I don’t think 100% is required. Usually it’s much better to increase the number outer layers.
That said, depending on the cost scaling, the possibly positive effects of extra mass, and how much I want to avoid a second attempt, massive objects can make some sense.

PLA becomes brittle with moisture (from your hands and/or air). I would recomment PETG / ASA / ABS (ascending order).

Yes, basically all CAD and slicers (3d-printer software) can mirror. PrusaSlicer for example can mirror and then save to stl again.

In addition to what TheYang said, I wanted to note that the spec for 3d printed parts for Voron printers is:

4 walls 5 top/bottom layers 40% infill

These are parts designed for 3d printers, so they have to be pretty good. They recommend ASA or ABS because of the temperatures that the printers get up to. Most plastic parts you’re used to are ABS or something like it. Water bottles are PET which is similar to PETG, though of course they’re really thin so they’re super flimsy that way. Your part wouldn’t be flimsy, but the plastic would feel like strong PET. That’s not necessarily bad, just different.

Personally, I would recommend trying to find someone locally, and buying them a roll of a filament to print the parts for you. I’d budget $15-30 for each roll, and each roll will print 2.2 lbs of plastic parts, which should be more than enough for your stuff and some failures, and probably leave them some extra for their effort.

I think you can probably find someone who is also excited to see that keyboard come together and wants to be part of the build. And I think trying to print it online is going to be frustrating because there could be little mistakes that make the parts unusable that you won’t know to predict because you haven’t done it before. Or maybe not. It’s a gamble, IMO.

I’ll bet you whatever the yen equivalent of a nickel is you can find some like-minded nerd in Tokyo who’d print this for you. Not necessarily a commercial service. Ask around local/Japanese language hangouts for 3D printing enthusiasts and you’ll probably find someone willing to do it.

• 20-50%, 4 walls (1.5mm+ thick)
• PLA should be fine, maybe PLA+. You can sand and fill both
• Mirroring is a simple button-click in the slicer, very easy to do
• no, but I’d contact a maker-space or find a person with one you can have print it. It will be cheaper that way. Maybe ask around on social media if any friends have one?

Try the JLCPB resin: https://jlc3dp.com/3d-printing-quote

For PCBway: For me, they are blacklisted. They screwed up my initial PCBl order by changing (without communication or disclosing that changes where made between quote and invoice) the quote/order. In particular, adding options that let the cost skyrocket without any additional value for me or even work for them. They tried a money grab.

the project only has the parts for one side. Is there an easy way to mirror everything ?

First make sure that you can mirror it to get the other half. If this is true download prusa slicer, drop the file in it, mirror, right click it and select save as STL.

Here’s my take on it, as someone who sells 3D printed things.

Infill percentage doesn’t matter as much as wall and top bottom thickness. My go to is 4 layers top, bottom, and walls. But for products I sell I usually do 25% infill, unless it’s very thin, like a stylus, then it’s 100%.

PLA is a good material for a lot of things. But for tactile things PETG and ABS are better.

And for the other two haven’t had the need to, at least not for a third party service seeing I have a 3D printer.