18.04 has two streams of life. (1) Server users (ten years of server-related updates) and (2) desktop users (about two years of desktop-related updates)
Server users can stay on the 18.04 release kernel for 10 years; it will be supported for that long.
Desktop users can join a refreshed kernel when point releases happen. It's called the "HWE stack" (Hardware Enablement) and also called "LTS Enablement, but HWE is how these packages are described so I prefer 'HWE'. Its primary focus is more modern kernels for newer hardware.
18.04.1 doesn't count ... it's just a bug fix a few months after 18.04. Not a new kernel.
18.04.2 brings the 18.10 kernel (after a suitably conservative delay to make sure it is very stable)
18.04.3 brings the 19.04 kernel
18.04.4 brings the 19.10 kernel
The trip stops there. If you want to stay cool and trendy, you are expected to jump to 20.04 which is the next long term release. Actually, 20.04.1 is the recommended time to upgrade.
The HWE stack is more than just the kernel. It includes updates to input device drivers, to the graphics stack and other such.
If you are impatient but still want to be safe, there are PPAs which have the HWE versions a lot earlier. E.g. the next proposed kernel is here:
https://launchpad.net/~canonical-kernel-team/+archive/ubuntu/proposed
This is different from the advice above to take a mainline kernel. The kernel in the proposed PPA is a real Ubuntu kernel.
There are also PPAs with upcoming graphics packages.
https://launchpad.net/~ubuntu-x-swat/+archive/ubuntu/updates
(and there is an nvidia proprietary driver PPA).
Getting the lastest desktop environment is not really supported though. Ubuntu and Kubuntu etc don't do major changes in the LTS.
However, if you like KDE (and it is very good) you can use the KDE Neon PPAs to get an always-up-to-date KDE environment on top of a stable 18.04.X with fairly new kernels (18.04.2 will go from the 4.15 kernel of 18.04.1 to 4.18; linux is up to 4.21 (now called 5.0) in the real world.
Summary: HWE updates make Ubuntu LTS releases a pretty nice place to be for desktop users.