That's what people living above ground have to deal with all the time. It's exactly the kind of challenge you miss out on when living underground. The way you talk about this like it's something new, I have to wonder if there are some dwarves out there that, because they stay underground, had never even considered the issue of stuff against walls being vulnerable. That would be a shame.
Is that
really excessive? With an above ground base, you have to defend a minimum of 4 sides of a cube. 5 sides if you're above ground and don't bother to break the ladders, wooden frames, etc. underneath you, and 6 if your roof isn't enclosed.
As for pathing bait, that's where bread crumbs could help. You got in there somehow, and you walked through air to do it. So even if you seal off the entrance, there's likely a weak side that's mostly air, unless you go out of your way to make things symmetrical, and hopefully the zombies can use that in their path finding.
You can hear zombies on the other side of terrain the same way you hear them on the other side of a wall, so you know they're coming. And as for not knowing exactly where they're going to break in, because you can't see through the terrain? That's called a
fair trade off for making them chew through a hundred blocks of solid material before they can reach you.
Busy work? Really? Is it busy work when a zombie tears down an above ground wall to get in, and you have to repair that wall and things above it? If not, what's the difference?
It occurs to me, TFP could spawn the zombie underground next to your underground base wall and just SAY that it tunneled from the surface with the dirt filling in behind it, and we wouldn't know the difference.
Broken legs are annoying, but then I break my leg plenty in PoIs, my own mines, or just on steep outdoor terrain. Better to address what makes it annoying in the first place, which IMO is the excessive length of time it takes to heal, even when using the best end-game option in the game to heal it.
With all that said, digging straight down seems fine. In fact it's how I've always been picturing digging would work - the zombie's go-to approach when it can't find a path and it's higher than the player. I still picture it as a specialist digger zombie instead of a horde trying to coordinate their animations, but the functionality is the same.