Hi
There are many restrictions on your system: You cannot build/remove it through robots, if you remove one side of an underground belt/pipe, you need to remove the other side as well, to restart from the beginning... and finally, you need to have xxx pipes/belts with you to create it
So, I have a suggestion: Changing the process behind the creation. Instead of building the whole underground belt in once, you could create "underground belts" items. Thus, those can only be connected to an end point or another underground belt, and they can be built one by one. Connection can be done directly between two end points or between one end point and an underground belt, then missing underground belts are automatically built, or set as ghosts if no resource is available. And same for the pipes.