This is why you are having problems, as i don’t believe it is a cartridge heated group, just a standard thermosyphon. In which case your offset should be between 10 -12.
If with your machine on its current settings, don’t change anything.Yep, just keep reducing the temperature by half a degree or one degree, allowing five minutes to stabilize and then pull water through the group. At the point it stops bubbling off to steam is the point at which the water is coming through the group at just under 100C.
Then change only the offset until your PID reads 99 or 99.5. If you have done it correctly, you will find that you will need an offset as something between ten and twelve degrees.