Same about reading the full post, I skimmed it. 2 thoughts:
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Have a look at LCSC.com, the buck reg and mosfets that you seek will be crazy cheap on there. To the point where you may want to consider the tried and true 24V method.
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It doesn’t sound like you are utilizing isolation from the 24V power anyway. Instead of relays and SSRs, can you just use a low side nmos switch to turn on the solenoids? Of course make sure to have a flyback diode.
In the end from LCSC you might find a 10cent buck reg that does the job and a bunch of 3cent fets and diodes.