Are you any place where it freezes? If the herbs/soil are left in the planter all year and you are in a freezing location, I would expect winter damage in the long run. Maybe not the first couple of years but it would happen. If so, I would use the planter only for 3 seasons at most and remove the soil/plants and cover it for winter.

To control the porous nature of bricks and how they would wick water away from the soil, I would probably use a plastic insert or liner of some kind. Obviously, there still needs to be drainage out the bottom.

And I must admit to being American and not thinking in metric. I had to convert your measurements to inches/feet. I'll post the conversions here so others may comment on the dimensions.

900mm tall = 35.433 inches
5m long = 16.4041 feet
300mm wide = 11.8110 inches

That's a lot of space for plants but I can't comment on the structural nature of your design other than the comments I already made about winter conditions - I live someplace where that is a consideration.

FataMorgana