When I was searching for suppliers for the 4-pole rotor magnet, ABM Magnetics unexpectedly offered to create the entire rotor during conversation. Another thing that the engineers suggested was to split the rotor magnet (which was 58mm at the time, before the new 7.2 : 1 reduction):

I communicated with our engineers and our suggestion is to divide the 58mm long magnet into 3 equal sections for the following reasons:

1. Because the size of the magnet is OD8, ID3mm, and length is 58mm, the magnet is easily broken during processing;

2. Dividing the magnet into three sections can reduce eddy currents and reduce the power consumption of the motor.
 

When the magnet was shortened to 42mm, I suggested that the rotor be made from 8 magnets total (2 equal sections of 4 magnets). They're also planning to install the bearings too, so the complete solution would look something like this:

They never mentioned price until today (I was thinking that it was going to be in the thousands but I'll stick around for the conversation anyway), but it's actually $16/rotor for 10pcs. I haven't asked about shipping price but I'm assuming it's another $50 on top.

It seems every motor option for "new" and not "new old stock from AliExpress" is going to be £20/motor and up, and the DIY Tetoroidiv probably would be no exception since everything else about the motor (3D print, toroids and coils) cost pennies. The 5 toroids needed probably amount to £1 total, and even when printing in £40/kg PBT, I'm not sure that the overmould would even scratch 10p in cost.

I'm also seeing if there is a possibility that I could acquire and use air-coils instead of having to coil at home.