I am searching for a approach to convert Bentley DGN complex shapes (without curve elements) into simple shapes

In the DGN reader, there is an Advanced parameter that you can specify to drop complex shapes. When this option is selected (ticked off), each part of a complex chain will be read as its own feature and no feature will be returned for the complex chain as a whole.

Have you tried using this setting in the reader?

I'm not sure if there are curves still present after dropping the complex shapes, but you could use the GeometryFilter and filter on Arcs to see if they are still present as a check, and then use an ArcStroker to stroke the arcs into line string.

Hope this helps,

- Andrea