LEDs for the suspended ceiling in the bathroom with 12 or 230 volts

Can I install a 230V dimmer as a flush-mounted actuator behind the light switch?

That doesn't make much sense. Just use a push button with an actuator right away.

What do you have to pay attention to with low-voltage spots so that they can be dimmed by the 230V dimmer?

That the manufacturer advertises or approves the function for the model.

As a dimming actuator, I was recommended the Eltako wireless actuator universal dimmer switch FUD61NPN-230V.

We have installed Eltako actuators in the sub-distribution panel and placed a push button in front. That works.

(Series connection of several spots)?

Really series or parallel? Because own transformer speaks for parallel connection.

Can I install a 230V dimmer as a flush-mounted actuator behind the light switch?

Depends on the dimmer.

What do you have to pay attention to with low-voltage spots so that they can be dimmed by the 230V dimmer?

Spots themselves don’t really matter. The important thing is the driver/transformer connected before. That’s what makes the difference.

Really series or parallel? Because own transformer speaks for parallel connection.

That is correct. Series connection only with central transformer and drivers designed for it.

That the manufacturer advertises or approves the function for the model.

Addition: The dimmer must of course also be suitable as writes. A dimmer via resistor will not work, a phase-cut dimmer rather will.