Using the tunnel as an entry way at all is considered a crouch base since they have to crouch.. i would expect this from someone else not you rohan smh lol
Even if you did the entry way slightly passed the crouching point, you can still kill the people crouching as they attempt to enter giving the base owner an unfair advantage. (which is what the crouch basing rule is trying to avoid)
Maybe if the tunnel had a turn in it, preventing that from happening. (aka if you weren't able to shoot them while they're crouched)
Also, This whole thing can be avoided if you just use the front door of the building as your entry way. Problem solved