I don't think you could put a such a deep dent in the metal border a wooden shield like that without causing the wood that it encircles to splinter or buckle and snap. Remember that if you take a section of it, it's coing to look something like this:
you might want to rotate the islands with wooden boards so the indentations between the boards run vertically or horizontally. pixels are arranged in rows so you should try and have lines laid out to follow them to prevent jaggy edges. this is probably the cause of the jaggy line seen in your previous image >…
Make the model from scratch. Sorry for my english :D Make 5 planks, not 1 mesh sculpted in 5 planks. then use trim dynamic with square alpha to break the edges. Use very negative fallof. Each part should be the separate mesh. That's easier to sculpt. For the "wood grain" use Orb brush, small size. Sculpt just few vertical…