Your motherboard does indeed allow a i9-9900K processor to run on it.
The question is how well.
When all cores are fully loaded the power draw of the processor can be considerable.
That power is managed by the Voltage Regulator Modules(VRM) on the motheboard.
If you look at the motherboard, you will see an area with 4 square components on the side, and two at the top. This is where the voltage regulation takes place.
Under load, it will get very hot; particularly with a 9900K.
Note also the 8 pin EPS power connector.
Compare that to the
MSI mag z390 mortar:
https://www.newegg.com/p/N82E16813144218
You will see more parts in the VRM zone as well as a substantial heat sink needed to cool them.
Your motherboard is MATX size. Nothing wrong with that, except limited choices.
Your case can hold a full ATX size motherboard.
As above, a good air cooler will direct cooling air through the cpu cooler and over the VRM area.
It will also direct airflow past a graphics card.
If you use an aio cooler with the radiator mounted at the top of the case, the airflow from the front will be diverted to the top leaving the motherboard and graphics card to be cooled less.
liquid cooling is really air cooling. The difference is where the radiator is located.
A noctua NH-D15s has two cooling towers with approximately the same radiator space as a 240 aio cooler.
The cpu cooling capability is comparable.
AIO coolers have other issues. One is that they do not last forever.
In time, air will enter the system making them unusable. Think 5 years.
And, while unusual, a liquid cooler will on occasion leak with disastrous consequences.
google "h100 leak"
Here are some images.
https://www.google.com/search?q=H10...DCM0KHUjiD1MQ_AUoAnoECB4QBA&biw=1580&bih=1291
Not for me if I can avoid it.
The original question was how to manage heat.
My suggestion is to keep your motherboard which is now working.
Return the H100 in favor of a noctua NH-D15s.
Your 9900K will do as well as it can with the parts you have.
And, that is considerable.