Ok, a few things going on here.
1. The game only supports 8 players.
2. Even in testing with extreme hardware, having more than 30 players will corrupt data and cause sync issues. For your hardware, you shouldn't be above 20.
3. 99% of game clients and dedicated server processes only utilize a single core. 7 Days will often use up to 4, but 95% of the processing is done on the main thread which is on one core.
4. Setting the max zombie count higher than 90 will break server performance. Especially when it comes to the AI managing entities.
5. This is a voxel game, your storage is your weak point. In my extreme test I had a full array of Optane drives pushing 45Gbps R/W. The max speed of your single SSD is almost a tenth of that. The more players you have moving around, the more data is being written to, and read from your drive. Most rented servers are using a pair of high-end NVMe drives in RAID.
For our groups servers, we're using a set of four drives in RAID. Max player count is set to 20, and we've been running up to ten servers on this configuration for years without an issue.
7 Days is not an MMO. It's made for small group co-op. You need to lower your player count, and lower your max zeds.