> ..Uh... From my understanding, a court declaring a statute unconstitutional basically makes the statute in question retroactively never a law.

It's not that simple. Even decisions on constitutionality do not always have retroactive effect, especially in criminal law where the court has an excessive fear of prisoners swamping the legal system with appeals and petitions.

> Precedent isn't something that only happens once you hit the Supreme Court.

No, but binding precedent is something only appellate courts can set, not trial courts, and it only binds courts under their jurisdiction. As a practical matter, overcoming qualified immunity at a minimum requires binding precedent, though theoretically it doesn't require any court precedent at all.

> Qualified Immunity basically gives the Executive one reusable get out of jail free card.

Qualified Immunity sets a very high bar. For one thing, it requires precedent to be "clearly established". Individual binding appellate decisions do not necessarily (or even usually) meet that bar, even with cases with identical facts in the same jurisdiction. Moreover, the officer must have "fair notice", meaning that even when the precedent is clear, if it's case law that doesn't get much media buzz or discussion among officers, qualified immunity might still apply. Generally speaking, it's really only SCOTUS cases, given their nationwide jurisdiction and high profile, that reliably clear the bar, but even then not always.

> Now, why we don't use it more, that's a lawyer question. I'm just an idiot who read a legal research book once.

You're probably thinking of 42 U.S. Code § 1983 civil actions. This was a Civil War Reconstruction statute that permits people to sue state officers, overriding state laws protecting them from suit. But it doesn't permit people to sue federal officers. There are various federal statutes granting rights to sue federal officers, but they're much more restrictive and narrower. In fact, in most of the recent ICE cases, I don't think the question of Qualified Immunity even arises; the relevant federal statutes either don't grant a right to sue ICE officers, or don't grant the right to sue them for the particular violations that occur. IIRC, most of these broader federal claims statutes only permit suits for property damage, etc, not for violations of civil liberties.