Woodburning pen advice needed

Hi,
I've been trying to use the woodburning pen here.

I've done many tests trying to burn a QR code, but several things keep ruining the tests:

1. After about 30-40% of the job is done, the pen tip stops burning. I am assuming it's accumulation of burnt debris on the tip. Can this be prevented/solved.. ?

2. The reliability of the Z height isn't good. Initial set up is perfect, and the pen burns smoothly on the surface, but then if I stop the job and go to zero and try again, the pen seems to be a little bit higher, so it doesn't touch the surface! weird...

3. If I set the Z height 0.5mm too low, to make sure that it burns the wood, the pen very easily is dragged too forcefully and it ruins the tip, bending it on the wood...

4. During the job, especially if there are circles or tight movements, the tip gets "unscrewed" and stops burning properly because it is moving around too much...


Any advice on these issues?

Thanks