Reduce the amount of water that enters the bowl after a flush
The bowl gets refilled by the tank after a flush. If you change the tube so less water goes into the bowl, then the water level will be lower.
You can also use an adjustable clamp and use that to adjust how much water goes into the bowl.
If that still doesn't work, you can drill holes into the tube so that some water leaks into the tank before going into the bowl. Obviously this is non-reversible, so do this only as a last resort.
You can also completely remove the tube so that the absolute minimum of water is in the bowl, but that may risk having sewer gas come through if the water level leaves air gaps in the toilet.