Finally! I've been trying to solve a solution essentially daily since the 8th and I've finally arrived at one that minimises the drawbacks and incorporates the benefits.

The main issue was mounting the 30mm cartridge in such a way that it didn't block an allen key to the plate bolts and that the cartridge can be held in a clamping mount instead of grub screws. When I got to save v45, I more or less abandoned mounting the cartridge horizontally since I couldn't think of a solution that didn't look ugly (and likely use a relatively large amount of material). 

My ceiling for modelling was 20cm3, and the result is a respectable 17.2cm3. The PCBWay autoquoter says this print is $35.

To cut down on material usage while minimising radiation, I've come to this heatsink-fin-esque design. The cutouts are perpendicular to the X axis to minimise air moving through the heatblock during printing.

Additionally, measures have been taken to both try and prevent leaks and prevent difficult cleanup should they occur. All inputs and output has a print-in-place o-ring to encourage deformation at the contact points, hopefully avoiding the need for post processing like the previous revisions. Then, leak channels have been added to allow any leaks a low(er) pressure way out, instead of a leak deciding to show up wherever. Furthermore, 5mm steel plates are used, with the mounting bolts as close as possible to the inputs to minimise flexure. Lastly, the design has a gap between the input plates and the cartridge+thermistor as a last line of defence against rogue plastic.

This solution has been facilitated by a modified input shaft and coaxialiser geometry. In addition to the split (hopefully) increasing flow rate, it allows the inputs to be connected without a bridge overhang, which would be the case if a basic L path was rotated 45 degrees. The coaxialiser is slightly more compact now. I did try orienting the coaxialiser vertically, but it uses more Z height.

 From the top of the o-ring to the Y split is 19.5mm long uninterrupted, so I'm expecting that this heatblock can put the "ate" in Coaxial8or when it comes to consuming filament.

As for aesthetics, I think I've done a passable job with the limitations I have to abide by. Like previous designs, there's a main element of symmetry. Somewhat inspired by earbuds, I've used fillets to make everything seem more organic, like a pebble. There are also some sharp edges, a design tip I saw used for cars (and some earbuds). I like how my logo is somewhat obscured, peering out of the shadows.

I just don't like how flat the bottom of the heatblock is:

[Oct 21] - I've opted to go with a sharp circle for all inputs/output. I feel it will have more pronounced deformation.

I'm also trying a new variable fillet across the bottom to improve aesthetics:

[Oct 27]

I've added a cutout so that the allen key has uninterrupted access to the bolts, requiring a slight aesthetics adjustment, and I've made use of the internal space to add a 5-way intersection so that each collection of 4 inputs can change their resultant mix relatively quickly.

I'm also testing to see if there's a step-increase in PCBWay's delivery cost if the shortest bounding box dimension is 30mm or more. The R1 and R2 orders cost sub-$8 to ship, which both were 29mm or less on an axis. R0 was 35mm at its thinnest and was around $16. R3 was 30.0mm, but I've reduced it to 29.9mm in hopes of reducing its $12 delivery fee.