I've looked through a lot of topics and tried to find solutions to the unit sprite overlay problem for a long time, but I haven't found anything suitable and similar to the behavior in the classic games like WarCraft 2, C&C, Settlers, KKND, Dune.
As noted, the main issue is this is an exceptionally complicated problem that even the designers of commercial RTS games struggle with.
This question comes up from time to time; see also my previous posts on it here and a bit of a longer explanation here (from way back in 2014!).
As I mentioned back then, I have spent many hours on this in the past. If objects collide with solids, they get permanently stuck. If they collide with each other, they get permanently stuck. If they try to steer out the way, they will hit something else doing the same and they both get permanently stuck. If you try to have smarter evasion strategies, it still gets permanently stuck. Often the evasion actually makes things worse and causes gridlock, and all moving objects get permanently stuck.
By far the easiest approach is to allow things to overlap each other (including actually ignoring solids, and only avoiding them when calculating pathfinding). Since Construct is aimed at beginners, this is by far the simplest and most reliable solution, which is why the template uses it.