I was trying to avoid along answer but if you are keen on making Alt as accurate as possible, here are some food for thought.
In commercial airplanes, they typically use an Altimeter to maintain altitude. An Altimeter operates basically on the same principle as barometer, just different technology. Depending upon where on the planet you are located, you input QNH (barometric pressure adjusted to sea level) for the local area. QNH is provided by the tower when landing, and its different when you are 36,000 ft above the MSL (Sea level).
Inside Pixhawk, you have barometer. It is not a commercial grade barometer and not designed to give 100% accurate results. Also, it is susceptible to temperature variance.
set TCAL_ENABLED to 2 (to learn and use calibration)
power down the board and let it cool for a few minutes
power on the board and leave it for about 10 minutes
learning will happen as the board heats up and while it is not moving
the TCAL_BARO_EXP should be updated with a non-zero value
optionally turn off learning by setting TCAL_ENABLED to 1 (to use but not learn new calibration values)
I will be playing around with this in few weeks so I can give you more lessons learned tips perhaps.
I don’t use $29 dollars GPS.
You can try something like this https://zubax.com/products/gnss_2 read the specs…