I finished the game once on easy with the Brains Dude (I think it's Mechanic or Engineer or something like that). (In classic, no dlcs back then)

I recently started a run with the Psi Monk and he seems overpowered. Summon Food? Heal? Burst Damge? Shields?

Sure, every class can put points their, that is not my point. It seems Psionics are way more powerful than the other skills.

It also seems like I have decripted every message there is. How do I get the other recipes? I still find tons of stuff I don't know how to use. A those messages bound to the level? Will I get new ones when I make it way downstairs again?

Seems like you are still playing on easy and on that difficulty you are able to decipher only the first 50 messages.

Psi based characters like Psion or Seeker have great offensive power but are very squishy at higher difficulties.
For them the real challenge are the first 20 floors. By this time you should develop enough powers to pretty much don't rely on other weapons at all :)

Psion especially has really tough start because most of his primary abilities are pretty low, most notably rifle, computer and electronics skills suck hard.
He really struggles to open basic containers and because of that I mostly died of food issues on Hard difficulty.
Once you manage to develop his computer, lockpicks and electronics skills to decent level then you can easily find a tough armor and some high damaging rifle or blade to support your Psi abilities. Ammo is not an issue and once you craft adamantium claws/sword you basically won.

So good luck on higher difficulties :)