I would like to bring something like this into a game engine. What is the most efficient way of retopologize this panel? Is Alpha map useful in this case? (This panel has thickness)
Make with polygons, alpha method is more expensive, DXT5 instead of DXT1 texture, alpha overdraw, and would look worse in low graphics settings that polygon version.
as you already have the modifier stack set up (not too firm with blender, but solidify gives a place thickness right) , just duplicate what you have and kill all the vertical and horizontal loops that do not add to the shape/silhouette, you might need to keep a few of those loops just to relef stress from the lowpoly, if you can do custom normals or face weighted ones you might be able to optimze it even further. but yeah just optimize your plane to hold the loops, then give thickness again and unwrap.
ideally you always have the lowpoly in mind when you build the highpoly. so a lot of your insecurities are moved out of the way from the start
Building on what Neox has suggested: in Blender you can run a limited dissolve and use the delimit option to preserve edge loops.
Here's a quick example where I've sharpened the edge loops I want to keep and run a limited dissolve with a sharp delimiter. Triangulate the mesh along shortest diagonals and convert tris to quads to cleanup the N gons and it's ready for any manual tweaks.
Depending on how you're going to bake it you could just use sharp edges along your UV seams or you could use a bevel weight and bevel modifier to add edge loops around the cutouts. A little minor cleanup with loop select and edge loop delete and you should be good to go.
If you do need to do some manual cleanup: vertex connect path is handy for connecting two points since it will automatically slice across any edges it encounters. Otherwise the knife tool can grab a starting vert and constant angle will keep the tool straight up or down which is quick and precise.
IMO (in this case) the key is to keep the cleanup as automated as possible before you go in manually adding or subtracting loops one at a time.
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ideally you always have the lowpoly in mind when you build the highpoly. so a lot of your insecurities are moved out of the way from the start