Hi Clement
When you simplify the model , hope you leave the PRESERVE PARTS on TRUE…
best if you use colorize on the original model and then simplify down for desired polycount
but if want have really good low-poly model highly recommend the Zbrush and its Decimation master, they have very optimized algo for strong decimation. But even there go with steps for final model, never go under 6% decimation there or get artifacts …
Its one of the best for “hard-surface” stuff.