If it were me, I'd try to isolate the faulty component as a first step. I can't quite make out what you are saying in the video so ignore this if you've covered these steps already.

First, remove your rear wheel and the brake pads to avoid contamination with mineral oil; next, disconnect the brake hose from the caliper. Then try to move some mineral oil through the caliper with your bleed kit. If you can't, you know the brake is faulty.

If the path internal to the caliper is clear, either the hose is blocked (unlikely) or the hydraulic module within your shifter is borked.

A total noob question: What kind of software are you using to design stuff like this? Some sort of CAD-software? Is it free/open source? I’d be interested to design a storage box to mount inbetween the water bottles in the frame triangle on my TT-X38.

Yes, CAD / 3D-modeling software. Commercial products such as Fusion 360 are very slick and can do a lot of thinking for you; licenses for personal use may be available. On the opensource side, FreeCAD* has been around for a good long time but takes some getting used to, depending on how your brain works. Either way, expect a steep learning curve.

* Ondsel ES is built on top of FreeCAD and to an extent takes the sting out of FreeCAD's somewhat spartan approach.

I believe you are on the right track. Whether an axle conversion kit consists of just end caps or end caps with an axle largely depends on the design of the hub. In other words, some hubs are designed in such a way that all you need to do is switch the end caps; other hubs will require more surgery.

Some hubs don't offer any conversion kits at all so you need to carefully research what options are available for the hub model you're interested in. I'd go with hubs from a well-known brand like the ones you mentioned as they'll offer a high likelihood of a successful conversion. As far as the conversion kits themselves are concerned, I am guessing the ones made by DT Swiss are pretty pricy so finding a generic AliExpress substitute is the preferred option.

My question is: Is that how this is supposed to work? When that screw is tightened, should that link rotate freely so that the B-screw adjustment can move the derailleur, or should it be locked in place like it appears to do on mine? I'm really a novice with it comes to derailleurs, so I'm not really sure what is more typical.

I don't have any experience with eGR specifically but in general rear derailleurs should be able to pivot around that 'L-TWOO Drive Systems' screw, yes. In practical terms, this is useful when you want to remove your (rear) wheel but the cassette starts jamming against the derailleur; with a free pivot, you can push the derailleur back, allowing for a clear path for the wheel to drop down. This may not be an issue with your particular frame, however - I guess it's easy to test.

I don't recall whether Yishun makes available their R068-D to the general public but size 59 is listed with a 597 stack and 404.4 reach.
https://www.yishunbike.com/product/r068-dperformance-road/

On the premium side, the Tavelo Attack has a 594 stack and 404 reach (size XXL).
https://www.pandapodium.cc/product/tavelo-attack-disc-brake-road-bike-frameset/

Another option may be to use a headset bearing with a 7mm width (height). If I understand the Ritchey diagram correctly, they specify a 8mm width bearing, so going with a 7mm one, your gap hopefully will be reduced to ~0.4mm.

Here is an example of a 40x51.9x7 bearing with 45° / 45° chamfers: https://www.wychbearings.co.uk/ACB519H7.html

If you think a 52mm one (rather than a 51.85 +- 0.05 one as specified by Ritchey) will fit, many more models are available; use the parametric search (specify OD as 52 and ID as 40 in the search parameters): https://www.wychbearings.co.uk/headset_bearings.html

For me, aliexpress.com links on this forum sometimes end up in a wonky redirect that doesn't work properly, while aliexpress.us links do what they're meant to do (it may be different based on your location).

https://aliexpress.com/item/1005006457498617.html -> redirect that ends up on the AliExpress home page
https://aliexpress.us/item/1005006457498617.html -> gets me to the product page