"To those 250 knots there is a corresponding dynamic pressure. As we increase in altitude, pressure decreases which means that the dynamic pressure would also decrease which means our IAS would tend to decrease as we increase in altitude."
As Alex has said, that could be true.
Let's imagine that we have a very unusual aeroplane that is fitted with an EASmeter, a CASmeter, a TASmeter and a MACHmeter.
With these instruments we could choose to fly at a constant value of any of these speeds.
Lets imagine we climb at constant EAS in the ISA below the tropopause.
When doing this we would see
Constant reading on the EASmeter.
Increasing reading on the CASmeter.
Increasing reading on the TASmeter.
Increasing reading on the MACHmeter.
Now if we repeat the climb, but at constant CAS we would see.
Decreasing reading on the EASmeter.
Constant reading on the CASmeter.
Increasing reading on the TASmeter.
Increasing reading on the MACHmeter.
If we now repeat the climb, but at constant TAS we would see.
Decreasing reading on the EASmeter.
Decreasing reading on the CASmeter.
Constant reading on the TASmeter.
Increasing reading on the MACHmeter.
This is the situation which you appear to have been describing in your most recent post.
And if we repeat the climb, but at constant MACH we would see.
Decreasing reading on the EASmeter.
Decreasing reading on the CASmeter.
Decreasing reading on the TASmeter.
Constant reading on the MACHmeter.
The relationships between EAS, CAS and TAS are determined by air density, which is determined primarily by static pressure.
The relationship between TAS and MACH is determined by air temperature.