From the looks of that picture, it looks like you just have too much "I" in the PID settings.
It follows well, but it follows with big jumps up & down.
Basically, "P" gets it there in large steps.
"I" follows it once you are there in smaller steps.
"D" slowly reduces or increases the I over time. (Use only in VERY SMALL amounts; one too many and it upsets the whole thing - especially if the "I" is already small to begin with.)
When I tune Boost control, I start with just some shop air and program a button (transbrake works well) to be a small amount of dome pressure (like 5 psi).
I set the boost control (use the boost vs time) to be a high number (like 25 psi).
Then I toggle between the 2 as I work with the PID settings.
Start with all at 0, bring the "P" up one at a time, until it comes close to the target boost (but does not overshoot) when you release the button.
Then you can start adding "I" one or two at a time, until it comes up to the target without overshooting too much or to quickly.
You will very rarely need the "D".
It's better to have it be a little slow & slightly undershoot, than to overshoot & oscillate a bunch.