While you're right that power bricks are generally crappy, linear power supplies are ... let's say questionable in their own right. Efficiency is downright horrible, typically in the 50-60% range, and of course they are huge AF, being essentially anti-SFF in every way. They do deliver incredibly smooth power though.
As gaming laptop manufacturers move to GaN bricks, there should be some decent options out there for high power 19V setups. And while bricks are annoying, you can always strip them down and rig some form of internal mounting. Optimal? Not even slightly. Doable? With the required skill and tools, yes. And, crucially, high powered GaN bricks need to use relatively high quality components as they will be concentrating a lot of heat in a very tiny box with no ventilation or even efficient convection cooling due to the plastic casing.
Also, thin-ITX is ... well, not dead, but an incredibly tiny niche. I doubt there's enough of a market to sustain any type of business on producing custom 19V PSUs for thin-ITX builds. And, in HDPlex's case, moving to direct 12V is of course a significant boost in efficiency and PSU simplicity, finally getting rid of the unnecessary dual voltage conversion setup of their previous systems. This is a great improvement IMO, as the vast majority of HDPlex builds do not actually use 19V. It obviously leaves those wanting 19V in the lurch, but sadly that's a necessity if one wants to improve the efficiency and compactness of HDPlex's setups.
As for existing 19V solutions, there are quite a few medical/industrial ~19V PSUs out there that could be used in a suitable case. Here's an open-frame 500W TDK-Lambda version rated at 95.5% efficiency in a 127x76.2x34mm/5x3x1.5" package (500W with a fan, 300W with convection cooling). Its ripple is a bit high at 360mV p-p, but my experience with these medical/industrial PSUs is that their ratings are very conservative. Here's a detailed datasheet. MeanWell's similar 12V PSUs often operate around half their rated ripple. And, of course, with any 19V system the motherboard will be doing its own smoothing, and ATX ripple/noise numbers don't really apply. Here's a semi-enclosed 450W model from CUI in the same form factor (slightly larger due to the enclosure, at 127x86.6x50mm/5x3.4x2") (and here's a variant of it that has a fan included), rated at 92% efficiency and 190mVp-p (datasheet). And, of course, there are quite a few other options - you just have to look in new places, like Mouser, Elfa Distrelec, and similar industrial suppliers. Another great thing about these industrial PSUs is that they come with detailed technical drawings, tell you exactly what connectors they require, and often provide .STEP files for including them in 3D models.
