Yeah that's a kind of a common issue with PC plates on hotswap PCBs. Not just on the GMMK boards.
My Keychron had the same issue.

PC is just too flexible to really get a solid grip on the switches.
And often times PC plates are slightly too thick which prevents the switches from getting fully locked into the plate with their clips.

it does care if the socket is worn out so much that the pins don't make proper contact with the socket anymore.
this is a common issue with hotswap sockets since they do wear out over time, especially if you are swapping switches often.

bending the pin slightly would cause it to make better contact with the socket again, solving any potential intermittent contact issues.

instead of bending the switch pins you can also press the contacts inside the socket together when there's no switch inserted.

Setting a different debounce or delay for a single switch isn't possible in QMK firmware (and neither is it in VIA since that doesn't do anything different from QMK on the hardware side).

You have one value in the firmware config that is applied to the entire switch matrix scanning.

 

And since OP said that the issue went away after bending the pins of the switches it seems that this was simply caused by a worn out contact in the socket.
And not from a firmware issue.

It's better to instead try to push the two metal contacts inside the hotswap sockets together.
There shouldn't be a gap between them when there is no switch inserted.

You can do that with a small screw driver or a needle pretty easy.

Bending the pins of the switch increases the risk of putting pressure down on the socket itself when inserting the switch.
This can rip off the sockets of the PCB and thus permanently damage it.

I would guess that this would be much more expensive than just CNC machining.
Especially with the size and geometry that some of the higher end boards have on the inside.

And it would probably still required at least some finishing in a CNC machine.
Screw holes will have to get drilled and tapped and the mating surfaces between parts have to be absolutely flat to not create any visible gaps in the assembled part.

And with casting the QC process is usually much more involved than with CNC machining since a casting can fail a lot easier than a CNC program.

Yes of course it's completely overkill for a keyboard.
Pretty much all keyboards around in this community are overkill.

But like you said for small scale production CNC really is the most sensible route if you want the case to be made out of metal.
Casting only makes sense if you are going to mass produce the product which doesn't apply to most custom keyboards simply because the market for them is too small.
No company in their right mind is going to invest tens of thousands of dollars into prototyping and manufacturing metal casting molds for a board that will maybe sell a thousand units at best.

As for the PCB:
Some designers have tried to do stuff like USB passthrough but many people here just don't really seem to want that kinda stuff on a high-end boutique board.
And since those PCBs are a hell of a lot harder to design and also more expensive to manufacture most custom PCB makers just decided to not go that route.

1. They aren't brands. Many of these boards are designed by community members like you and me and are just sold through the known vendors to make it easier for the designer.
   That's why you won't necessarily find a website for all those names.
2. Yes a lot of these boards are limited run items, simply because it's not economically viable to mass produce such niche products. Production of them is very expensive and the market is very very limited. To keep production costs to a somewhat manageable level for both the designer and the vendor they are only made in preorder batches.
3. It's not "just aluminium". They are CNC machined components that often times have very complex designs.
   CNC machining is extremely expensive, especially for low-volume production like it's usually done with keyboards like this.
   And even raw aluminium prices are currently through the roof due to the ongoing economical problems.
4. In some cases there is artificial scarcity and price gouging (like with Keycult or TGR boards for example).
   But most of the smaller lesser known groupbuys are absolutely realistic prices.
   As mentioned CNC machining for low volume production is expensive af and mass production is just not reasonable for a product that has such a small niche market.

​

If you want fast availability and shipping the super high-end boutique keyboards aren't for you.
Those are more something for collectors that have patience and no problem with waiting for a product for a year to get something unique.

And you honestly don't have to get one of those if you just want to build a good custom keyboard.
There are quite a few easily available options now that are still very much high-end.
This hobby has seen a rapid growth over the past two years and there are quite a few vendors that produce boards now that are good quality and usually in stock (or at least restocked frequently).

KBDfans, Keychron and Cannonkeys for example all have several beginner oriented keyboard series (both alu and plastic) that are easily available.