Can Congress not employ its own lawyers and investigators with subpoena power?
While I’m sure this is a rhetorical question, the reality is that excessive executive overreach has been institutionalized by every president since FDR, with the President having broadly unassailable authority over wide areas of government control. And it has been exercised by every president regardless of party, sometimes out of necessity, to deal with a stalled legislature or to act ex nihilo where no existing law covers an area of authority, but often out of political expediency. And aside from judicial reversal, there is really no remedy available to the legislature to correct it (see the War Powers Act and how every President since Reagan has utterly ignored it at their pleasure).
It is not the President’s “determination as to how justice is to be pursued…”; the President may appoints the Director of the Department of Justice who is supposed to have broad authority over what is and is not worthy of prosecution. The President has certain protections against prosecution outside of impeachment for actions taken in the official capacity of the office but is not immune from prosecution for private actions taken before, during, or after holding office (although there is some limited protection against using prosecution to deliberately interfere with the executive authority).
In the case of allegations of financial improprieties with campaign finance, unreported contacts with hostile foreign interests, and collusion with same interests, simply being President does not grant immunity in perpetuity, and using executive authority to interfere with an investigation is obstruction even if it is within the ostensible authority of the office. Whether a Republican-dominated Senate would ever hold him accountable, on the other hand, is another issue entirely.
Stranger
Not true, he most certainly CAN be impeached on uninvestigated crimes, or on imaginary crimes, or on no crimes at all. Congress can impeach the president, period. There is AFAICT not even a requirement that they explain why.
Of course, if that was actually done, the results would be potentially very frustrating and weird.