Trimmers and Buck Converters

I have a question about the risk of using, a buck converter module which uses a trimmer to adjust its output voltage, on the long run, for 24/7. I understand that potentiometers deteriorate over time, and if it does, I assume it will alter the output voltage as a result? Are fixed resistors used in such cases, instead of trimmers?

Depends on which type of trimmer your buck converter has.

If its the minature open skeleton type, then yes they do concern me too that dirt and movement could alter their critical setting.

If its the typically blue coloured multiturn trimmers then have found them very stable over the years of use in many types of sensitive circuits.

You could replace the skelton types with fixed resistors (smd) or a multiturn if space allows.

Generally I will always go for the blue multiturn types.

Either way if its still a concern for you , build a simple Crowbar circuit so if it does go over your have protection.

The multiturn ones are usually cermet (ceramic-metallic), not carbon-composition, so they aren't affected
by moisture and oxygen from the air (and are better enclosed from dirt).

I like to use these things in combination with an ldo. Get the voltage down to a little more than Vcc+dropout and burn the rest.
For my ESP, I made a board where the LDO circuit in smd is on the pcb, an mp1584 above it and all of that under the Wemos D1 Mini.