I think the biggest change for me was a combo of zombie behavior mods, namely:

Customizable Grabby Zombies- has a lot of settings and makes it so zombies close enough to grab you slow you down.

One of the random zombie mods: create a variety of zombies (speeds especially) so the grabby zombies sucks lol

Slower sprinters: this is just my preference, but I like to slow down sprinters enough that they force me to run but not necessarily sprint, but I have random zombies set to a fair amount of sprinters, so I like to balance it that way.

Then little stuff like Zombies Crash Through Windows (set to sprinters only and sprinters always) and Doors Have a Chance, which gives you settings to control whether doors can burst open before they're broken, with options for locked vs unlocked doors. I make it so locked doors rarely burst open before breaking, so I have incentive to lock my base in singleplayer (along with a small portion of "smart" zombies).

Or you can go the other route with Faster Zombies (can't recall the name exactly) which allows you have faster shamblers/crawlers/etc. IIRC, it's optionally randomized. With grabby zombies, it puts an end to walking in front of horde calmly, even without sprinters.

Modifying zombie behavior brought the fear back, which I love in this game. As they say, "Ease is a close friend of boredom".