I agree with you, more or less: as it is now, after the early game, for static survivors(staying in one place), there isn’t a path of narrative tension. Travel brings it in terms of what you can find, and there are places that can kill you if you’re unprepared/uncautious.
Also, if you keep fighting, your melee combat skills will get higher and higher, and so combat gets easier. Not to the point where doing more damage faster makes it to be a bore against everything, but zombies really aren’t a challenge after a while.
I was in a position similar to you for a while, and was going to start editing the JSON to create new monsters and buff the already in-gam, but my character died exploring a lab, so, yeah. Maybe editing the JSON can make hordes a challenge for you, perhaps? Also, you can edit world values in the options menu, that includes the zombie Spawn multiplier. Both can be a temporary solution while we wait for more paths of narrative tension.(or using the debug menu to Spawn nasty stuff :p)
For now, i agree with Valpo suggestion: if the world was alive, if stuff could happen in a macro-scale, problems SURELY would reach your fort. Merchants, bandits, triffid hordes and so on. Suggestions to how it could be solved? I will think in something for a while.