Clemens,
The hardest part for me is trying to describe the filing effort. I file the bottom so it has a 19 or 20 mm wide flat area on the bottom. This leaves 2 mm or so of gently rounded edge on each side. I do not file the tip.
I use a coarse file for aluminum or steel. Be aware that aluminum galls, and it will be easier to smooth with sandpaper if you clean small chunks of aluminum out of the file. An alternative would be to use an aluminum wheel on your grinder – and go very carefully.
I smooth the filed area with 220 grit sandpaper, followed by 360 or 400 grit paper and fine steel wool. Then I rub in on wood or paper to see if it is still rough and leaves aluminum marks. I don’t want to leave marks on the yellow fabric!
When done I can use the tip with a raised handle position – and I do this very frequently! Then I slowly lower the handle to use the full length of the bottom. I get a fair way down a seam before the iron cools.
On seams I can tilt the iron to one side, rocking the rounded edge over the tame edge and squishing the TPU to the edge of the tape. Not always easy to see with black tape, it is more readily viewed with seam tape made from red fabric! IMO.
Hope this helps.
Bruce