Off the top of my head:
Star nut installation tool
Head set cup press

Sometimes it's hard to know until you need the tool.

That's basically my experience, I cleaned the pedal threads up with a tap.

Short answer is that if you want to be making parts that are reliable your price of entry is going to be a $30K printer. If you are willing to check them periodically and live on the edge, you can just use what you have. Like any part made with additive methods, voids and delamination are problems to manage.

I have made tons of reliable, functional parts on my bikes (mostly mounts) using my 250$cad Bambulab 3D printer (and countless things around the garage and the house). While I wouldn't rely on a printed item for something that would make me crash if it failed, I have no issues with other parts. My A1 mini was super functional for the price - PLA+, PETG, and TPU were very printable and can make functional parts. Upgrading to printer with an enclosure is worth it to be able to print ABS - it has a much better tolerance for heat. You can get around a lot of the limitations of printed parts with good design (make things thick), and choosing the correct print orientation and material.

At work we recently got a couple Bambulab H2 printers for prototyping and tooling. We have a machine shop, so 99% of production stuff is machined, but for fixtures, Jigs, and even functional prototyping, 3D printing works great. We still have a ~30k printer that we bought a couple years ago, that can make stronger parts, but 95% of the time I go to the H2S/D printers for their speed, size, and convenience.

I'm not exactly "into" 3D printing - but a lot of my hobbies and work benefit from printing, and so far my experience with Bambulab has been plug and play, super reliable and shockingly cheap. I've had many paper printers that required more fiddling/hr than my 3D printers.

Anyways, I think now is a good time to get into 3D printing, as the tech has reached a point where you can use it without having to fiddle with it. The Bambulab P1S combo is on sale for a very impressive price right now, but I've also heard good things about Elegoo, and Flashforge. I'd ensure the printer has auto bed leveling, and can print the filaments you want.

There are still some 26er QR frames available on aliexpress, however I'd go used for what you want - there were frames with your specs in the early 2010s. I'd look for the more boutique hardtails like Chromag, and Ragely. I think Kona also had some stuff in that era - something like the steely.

Otherwise looking for a 27.5 inch frame, with a higher Bottom Bracket may be fine too - you can compensate for the lowered BB with shorter cranks.

Pointless. Most people don't even need 4pot brakes. It is just more moving parts and seals to fail.

Agreed, generally, more pistons just add thermal capacity, braking force is more pad/rotor diameter dependent (Though variable leverage like servowave can also change this). I've ran 2 pot calipers on on DH bikes before, with semi metallic pads and enough rotor it isn't a problem.

I only run shimano brakes these days cause they are so reliable, and so easy to bleed. I have a set that I've had for over a decade, only needing 1 lever bleed.

In My opinion, best value for brakes:

Shimano mt200, I can typically get these for 40-50$ CAD, replace the stock pads, and these are very capable. The lever is a not ideal for 1 finger braking, but it can be adjusted inboard to work fine.

Shimano MT420, I can typically get these for 90-100$ CAD, I'll spring for these for bikes that will see more lift service, or if their for someone with small hands (the lever is a bit more comfortable)

Those Cranks have a really narrow Q-Factor - I have them but haven't found a frame that frame that they fit yet. But I've only really tried on MTB's.
I have a couple other sets of cruzbike cranks that have held up really well though, but they aren't the cool machined look.

I'd be skeptical about the fork - are there good Chiner forks yet? I would look for something used - most older fox/RS/Marz are pretty trivial to rebuild, and there isn't much of a gap in performance, with the middle-ground modern stuff.

Stamped steel cogs I've come across tend to be too thin and eventually flex out-of-round.  Shouldn't be an issue with a nice steel cog, just the cheapo ones. 

Dunno if they're the same as the ones you are talking about, but I've had no issues with MUQZI branded cogs. They are of the narrow base variety, so only use on steel freehub bodies. But they seem round enough (always hard to see if it's chainring or cog), and I've put some decent distance on them. They were the same thickness as my DMR COG.

I've also used those RiSK ones with no issues - in 12 speed, and 8 speed.