Is it possible to reconfigure what a device would do as a "dumb device"?

In the kid’s room, I’ve got a light switch with a Fibaro dimmer behind, all wired to main power. I’ve also got a dimmable lamp in the corner that’s currently not smart.

Let’s say I’ll buy a smart bulb for that lamp. Is it possible to program the fibaro dimmer im the following way:
If time between 7-19 o’clock then behave light thw normal switch for overhead lighta
If time between 20-6 o’clock then don’t switch overhead lights, but instead lamp with brightness 50%. Double press still switches over light.

I know I could switch the lamp as an additional action, but can I do it instead of the dimmer’s normal function (to switch the overhead light)?

No. There is no way to decouple the dimmer switch from the overhead dumb light. it is connected via wires. when you switch it on the overhead light will always come on.

You are able to make it come on at a very low brightness though, as it is a dimmer. Some dimmers remember their last sate so it may flash bight briefly before going dim if the last sate was bright.

Change the overhead to ‘smart’ as well?

Ah that’s what I feared. It probably would’ve been trivial if it was simply a light switch not connected to lights via wires. So I guess my only options are:

  1. put in another switch that’s not wired, it’ll switch the lamp via zwave/zigbee
  2. disconnect the wires from the switch and overhead lights (would need to make the lights smart then to be able to switch them
  3. do this weird workaround with overhead lights turning on dimmed

Maybe I could de-activste the stabdard double press function and have double press switch lamp instead?

Actually if you would consider replacement of fibaro with shelly it would be possible to achieve what you described (at least with Shelly 1L or Shelly RGBW). In these modules it is possible to disassociate button inputs from the switch/dimmer, so you can just take button input, manipulate it through the HA and act appropriately on the lamp (either viaq build in switch or lamp having smart bulb).