How do I close the gap - under the fire protection door

Hello everyone, how do I best fill this gap? The gap is about 1.5 to 2 cm high. It is located under our fire protection door at the transition between the utility room and the garage. When I step on the track, there is of course movement. Critters and dirt always accumulate in that transition. I want to stop that now. I initially thought of silicone. But the gap is quite large. Then I thought of compressible tape. But in terms of critters, it is not "sealed" enough for me.

I would mortar that, if necessary, whitewash it afterwards. A remaining lime cement plaster would also do.

Of course, there is movement when I step on the rail. Bugs and dirt always accumulate in the transition.

Of course, that is not the case. If something moves there, the frame was not installed correctly. The area was also not plastered before the garage was paved. o_O

Can you put rock wool in there, it insulates a bit more (cold bridge and then colored silicone in front, which compensates for some movement)