22 gpm is way too much for an 18 hp motor. As a rule of thumb, it takes about 1 & 1/4 hp to make 1 gpm of flow at 2000 psi. Mathematically you need 26 hp to flow 22 gpm at 2000 psi. Right now you have LOTS of flow, but not enough horse power to make 2000 psi. The pump must be sized in relation to the motor. Too big a pump = lots of flow, but not enough pressure. Too small a pump = lots of pressure capability, but not enough flow. Decide what psi you want for your system, then plug in the numbers and do the math.
For a wood splitter, the best of both worlds is a two-stage pump. A large gpm pump for fast travel speed under light load, with a smaller pump stacked onto it to generate the high pressure needed for that last stage hard push.
https://www.calcunation.com/calculator/hydraulic-pump-horsepower.php
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