I also have (dim) interest into this direction. I’ve seen KiCad being (ab)used for things like electrical cabinet layout to hydraulic and pneumatic circuits. There is more specialized software for this (Such as for example QElectroTech) but if you only do such diagrams occasionally then using the software you already know is an advantage.
There is a library on github for hydraulic symbols, but unfortunately it mostly has “exotic” symbols. I collected some info about ISO1219 and then I sort of got lost somewhere between an overwhelming amount of possible symbols (especially for valves and their actuation methods) and paywalls.
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