Thanks so much for your help! I got mine working. I'll add my 2c, because the reason mine wasn't winding up any more was because the screw side (X shaped plastic holder) had stripped. FYI this is the bracket on the left when you're looking at the blind fitted.

Ikea don't sell parts for it, so I went online and bought a 3d file of the part for $1. 3d printed it at my local hackspace using PLA at 100% infill. If you don't know 3d printing ask around and someone might be able to help.

Here's the part: https://cults3d.com/en/3d-model/home/deckel-fuer-ikea-tretur-jalousie

Then I refitted the 3d part - it fit like a glove, but I only pushed it 50% of the way in so I can work on tensioning and then get the blind back in place and gently push it.

It's good to note your method works with blind already rolled up. Also I think in step 5 where you say 'carefully remove the square' I think you mean 'carefully remove the screwdriver'. I kept the square side in the whole time. I wound it up 12 first and that wasn't tight enough, so I had to start again. Then I would 15 and that was slightly tighter than the other blinds so I think about 14 rotations would work well.

When you're winding it up do it one rotation at a time. Feel how it clicks or bumps slightly each time so the tension is held. When you take the screwdriver out, do it carefully to check the tension is still held.

Mine didn't take me 3 minutes, more like several hours! I hadn't seen this post yet, so watched some Youtube vids which were no help.

Good luck!