So it started when I replied to a comment (that itself replies to me talking about this project):

Trying to set up a simulation

I was thinking that it would be as simple as setting up an electronic cooling simulation in Fusion360:

But then I go to solve and:

Thus, I started looking around for another solution.

Trimming down the mass of the hotend

I realised that I could use a smaller elipse for the body of the hot section. I also tweaked the thermistor cartridge so that less of the hole depth needed to be tapped:

This got the mass down to 220g. It also means that the bolt on the X-gantry no longer is a problem:

Trying ElmerSolver in FreeCAD

The main one I stumbled on was ElmerSolver in FreeCAD:

I soon realised that Elmer was the software I was looking into for Tetrinsic too. Anyway, I imported the .step into FreeCAD and noticed that it looked slightly longer:

I decided to move on from that mildly interesting observation and start messing around with features until something exploded:

Looking for other solutions

• There's Simscale but it seems that the free teir only gets 10 solves and there was no mention that they reset after a certain amount of time.
• There's OpenFOAM and I'll have to look into that, since FreeCAD supports that too. 
• There is Simflow that is based on OpenFOAM that seems to have a very nice-sounding free tier 

Further design optimisations

I asked Elelven about any price reduction strategies and she, once again, suggested to skip on the tapped threads:

I thought that it would be a good idea to actually modell the important threads into the part, which are all the M8x1 threads and the M6x1 thread at the bottom. I watched the first strategy in the below video and looked at comments, and decided to offset the faces by the equations below the video.

tol = 0.3mm
Angled Faces Offset:   -tol / 4
Vertical Faces Offset: -tol / 2

 Lastly, I deleted the bottom-most face so that there wouldn't be a 90 degree overhang where the thread meets the internal chamfer:

Then I did the M6:

The next thing to do was to remove the flat face that would require the printer to bridge 90-degree overhangs and replace them with 50 degree overhangs. To get the starting chamfer to work, I first had to offset the eliptical face:

Then I could do some more DeleteFaces magic to obtain this:

Lastly, I pushed the below face down to save around another 3 grams. The result is that the autoquote is now $167.17, over $20 lower than the first version I sent to PCBway.

The end result looks a bit more interesting now:

The bounding box (compression fittings to nozzle tip) is 49 x 49 x 124mm and the printed mass is 212g (down from 241g from the first version I sent to PCBway), which is in the same ballpark as 10W+ (output) laser diode module sizes. Unfortunately, it seems this hotend is going to be in the same ballpark in terms of price too.

[08:40] I just got the price and it's a whole $194.00, so the additional cost for tapped threads is likely the similar $30-ish that I experienced back when I designed and submitted the printed coaxial heatblock. 

[20:40] I also looked at the additional fees involved. Fedex is suprisingly the cheapest option, even lower than others like China Post or PostNL. At an optimistic best, the entire cost is £182, but could be as much as £182 + 20% duty + £12 Fedex fee = £230. 

[Jan 05] I found the issue with Fusion360; I needed to change the cloud credic preferences.

I then proceeded to try and set up a thermal study, trying to get over the hurdles along the way:

I followed the suggestion of symmetry, as per the Fusion360 docs:

Then alculated the convection coefficient:

(11.89 * 0.0004719474) / (1092 * 10 ^ -6) = 5.1386946758 m/s
10.45 - 5.13 + 10*sqrt(5.13) = 27.96950330581225 W/m^2-K

 And this is the result for a nozzle-cartridge temperature of 250C and coaxial-cartridge temperature of 180C:

As you can see, the heatsink is mostly ineffective.

I tried a fin-based design and it slightly improved things, but still nowhere near good enough. It seems that "it's over.", with the only mitigation strategy I can think of being to reduce the amount of material that makes up the vertical tubes by modelling a mesh design.