Craftbukkit basic server help

Hello everyone! I'm planning to deploy a private 1.15 server for me and my friends to play, but we won't mind if anyone else will join
My questions are:
1. Which plugins should I use for better performance? We had some issues on 1.14 and don't want them to ruin game experience again
2. Which are the best plugins for security? Anti cheats/hacks
3. Are there any useful to have plugins? We'll have a vanilla Minecraft server without any game modifications
4. Previously I used a DigitalOcean VPS with 8GB of ram and 4 vCPUs, but from my diagnostics I saw that 5.5GB of ram allocated was working best and only one of cpu cores was used. I'm wondering if that's because of Minecraft limitations or if I configured something wrong. My question is what the best config for 10-20 players maybe there are some good servers out there?
Sorry if some of questions are dumb/already been asked, and thanks in advance for your responses

 

4.Minecraft only needs RAM! The processor it uses is not particularly=)