Installing kernel 5.1.x will given any improved results?
Most certainly not. I believe you are in a mistake here.
Having 20-40% CPU usage when idle is completely normal, there are background system processes that are working and are consuming processor cycles. They use that much because the system is mostly idle and no other processes are using the CPU, so they have the freedom to use as much as they need. This is expected behaviour and it's completely fine. What you need to look out for is the system load, as long as that is low - everything is fine with your system. You can't expect CPU usage to drop to less than 20%, just because you are not doing anything on the machine.
So do I wait? How can I get notified about kernel new version?
Or I install from outside? after installing, I will get update for kernel?
I suggest you wait, because a kernel from outside is incredibly tricky, unstable and risky - it could very well break your system.
And as long as you update your system regularly - you will receive the new update whenever it comes. Debian cannot notify you automatically about this, you should just run
apt-get update daily to see if there are any new updates (you can list upgradable packages with
apt list --upgradable).
Debian systems are made to be extremely stable, so we, Debian users, receive updates later than most other distros, however, it is for the best if you want a stable system.