In my opinion, having a high cholesterol level would predispose you to high blood pressure, which would be the issue.

Since you just started running 1 month ago, the reason you could barely last 2km is due to a poor aerobic base. Just keep running & you'll improve.
As always, a combination of aerobic & resistance training would be beneficial.

eh.. blood pressure & heart rate is different yea.

High cholesterol may lead to high blood pressure which leads to other risk such as atherosclerosis or heart attack.
Regular exercise may help improve your cholesterol level.

Your heart rate spiking with an increase in physical activity could be due to various factors, for example a poor fitness level/cardiovascular system.
Think of it as your heart needing to pump more (thus higher heart rate) for the blood to flow through your whole body.
The more you exercise, the stronger your heart will grow, thus able to pump more blood -> so your heart rate would spike less (for the same intensity that you're comparing to).

Are your legs strong? As in do you undergo strength training or even tempo runs?
But why stride length specifically?

As mentioned above, overstriding leads to braking forces therefore waste of energy.

Feel syok. No real benefits.
There are better ways to "improve blood circulation", like the good ol' walking.

If you're running to build bigger muscles & strength, then you're barking up the wrong tree my friend
Strength training with progressive loading is what you need to build bigger muscles & strength!

Steady-state jogging/running is for your cardiovascular system!

Training different properties dude.
Endurance athletes undergoes volume overload induced cardiac hypertrophy, whereas strength athletes undergoes pressure overload induced cardiac hypertrophy

Endurance athletes have larger left ventricular volume, whereas strength athletes have thicker left ventricular walls.

The dizziness you feel after an all-out effort is because of a lack of oxygen to your brain, nothing more.
Your heart rate will hit 185+ for a brief moment and drops immediately as you sit.

Yes, 1 will lose the endurance built from running after you stop running. Just like how 1 would lose strength after 3 weeks of not lifting.

For building strength, strength training is definitely number 1 but
in terms of cardiovascular health, strength training is probably not number 1.