I took a look around for high-ampage 12v DC power supplies. There's a few out there which do 12v at 12.5a and provide a DC jack splitter (https://www.amazon.co.uk/12V-12-5A-Power-Supply-Replacement/dp/B0BQJ4B8FT), but that's using a brick power supply.

The reason I'm keen to get rid of the power bricks is due to the space constraints I have. If I can replace 4 devices bulky power supplies with 4 USB plug ones (like this https://www.amazon.co.uk/gp/product/B08CDDSVN4) then I'll be happy.

If I had more space then I would absolutely go for something like this: https://www.amazon.co.uk/Surom-Universal-Regulated-Switching-Computer/dp/B072J97N8T and custom-do my cables.

Yeah just reading up on it. I'm wondering if there's a USB-C device which has zero negotiation time? At the worst case, I may just get a dedicated USB-C charger for the Router as that has the most obnoxiously large power brick.

12v 3a is 36w, which is what all the devices I've linked cap out at. Most of the devices have a power brick which puts out 12v at 2.5a.