A fun (Difficult), 1.20.1 Minecraft modpack with tons of mods for an intense unending zombie survival:

Currently in beta* (Version 1)

Setting:

You, and your potential colleges, wander the empty landscape you are the only ones left all other civilizations are gone only their ruins remain and their reanimated corpses, not to mention the weather and apocalyptic events that wiped many to begin with. You learnt early on these were no normal zombies they could always sense you through brick and stone they would always know where you are, with some sought of 6th sense really. They also learnt how build and breck a month in killing many of the few survivors. Now it's just you and your potential colleges. Good luck.

SETUP (These are all optional & can be changed):

Firstly , add Tissuou's Zombie Pack (May cause a crash when first added):

Do the same for "fresh animations" & connected textures

Next, create a world with the shown custom city type:

If your creating a server with this next ignore the last two and export the world file and load it into the server then on the server console or in game do the next 2 steps with (/menu & /apocalypse)
Then, configure the zombie ai (In game):

For a harder , more intense experience leave zombie timed mining off

Then, configure the apocalypse type and length of time until it happens (In game, requires operator todo) (Warning laggy):

Finally use the command /start

Extreme zombie survival (better zombie ai)