Question about Erard Standit 400

In principle, yes. However, with a 55 inch you are at the upper limit. I also ordered the 400 first, but then changed to the 600 model (more flexibility in height adjustment).

https://www.erard.com/en...

There is a picture with a diagram in the product images, the AF 9 55 inch is 8.6cm thick, and weighs a little more than 23kg, so according to the diagram you are already off the chart and therefore the StandiT 400 is not suitable, so have a look at the 600 dan (should there also be such a diagram for the 600 on the Erard site).