I’m a lot like you in that Minecraft never held my interest but that is a precursor that many people liken to this game.
Yes, At Day 21 you start to see the Ferals and the Cops with their vomit and it does become much more difficult. If you can manage it, it does pay to make better fortifications than most of what you find.
But my ‘go to’ by then if not Day 28 is to shore up a Police Station for horde nights. Block off the stairs and put an inner ‘lip’ around the interior courtyard. Make a few exit ladders from the top floor. You can hold out forever upstairs in one of those and pick off or blow up the horde at your leisure. It’s a little cheap but I sometimes don’t have much use for making my own fortress.
Yeah, I purposely avoided much chatter about it but you’re right; it’s much more important by about week 3 or 4. There are many other videos out there with ideas for fortresses and of course reinforced concrete and steel are the ways to go.
Day 35 and Day 42 are the worst and I think after 42, the day 49 horde resets to a humble Day 7 horde. So it seems to many like the game is meant to be ended after surviving Day 42. Of course you can continue as a sandbox exercise as long as you like.