I don't believe the approach the IRS takes is to set targets and only audit the lowest hanging fruit up to some target. They have different sub-organizations pursuing different goals, and some sort of vision about fairness that means going after tough cases.
> This exactly why ICE agents tends to target illegal immigrants that actually get a job and contribute to the society instead of criminals. Because the former are easy targets.
This is completely orthogonal, but also untrue. It's way easier to go after criminals, as long as states cooperate. The recent Trumpian ICE is more expensive and less effective than earlier regimes.
> adding more IRS agents only cost more tax payer’s money and give regular people more headaches.
Many, many regular people underpay the taxes they owe. Additional IRS agents help close the gap between taxes owed and taxes paid, at a cost lower than the additional revenue. Your argument is just "individual tax cheats should be able to get away with it," which I can't agree with.