I am making some progress. I managed to get the "out of vision"-bug fixed, kinda. I tried the method of searching for destructables in a rect by the order point, and at the start of the game, it doesn't work. I made a counter to see how many destructables are in the rect, and it always turns up zero.
After some time though, and i don't know why, i can target trees out of vision just fine, and the worker goes off harvesting like normal, but the same does not go for gold mines or stone mines. I dunno if it has to do with their pathing being larger, but i am testing that now.
If it has to do with this, then there might be some problems with resources that have a large pathing, since i would have to expand the search range. If the search range is too large, harvesters will start harvesting resources in range even if you just told them to move next to it, and that might be annoying. Also, the counter i did still shows up "0 destructables found", even in the case of the working trees, but that might just be my counter that is broken.