There is no constitutional court other than the Supreme Court.
All other courts are established by Congress. Congress has the authority to establish them however Congress so desires. That is the text of the Constitution. And it is the text that is binding. One can imagine that the text we have, and not the text we imagine, was selected for specific reasons.
1. Congress initiates and structures (ordains and establishes) everything about all inferior courts.
2. The Constitution calls for a Supreme Court only. It ALLOWS for other courts under the direction of Congress, and even these are not necessarily permanent. (”time to time” they are set up.)
3. The Scotus justices are subject to good behavior tenure. The Constitution is silent about the tenure of Scotus justices. Presumably, the Congress could pass a non-reviewable law limiting the terms of Scotus justices — GIVEN that silence.
4. Congress clearly is the body that structures all other Federal Courts. It can structure them with built-in tenure. That would not in any way violate any “good behavior” requirement.
Interestingly, the justices of many State Supreme Courts are elected officials (my own included.) These constitutions go back to the generation immediately following our US Constitution. Apparently, any logic regarding the “independence” of the judiciary being tied to lifetime appointments were neither compelling nor binding. This is interesting in that those constitutions so often reflect the national constitution in most other areas.
No, it's not silent. It states that they shall (mandatory) hold their offices during good behavior. This is very plainly a lifetime appointment. It's been considered a lifetime appointment since the Constitution was drafted, Hamilton in Federalist 78 explained that it was a lifetime appointment, and the Supreme Court has held, repeatedly, that it's a lifetime appointment. There's no debate here.
It can structure them with built-in tenure.
No, it can't. That would violate Article III. Again, Hamilton addresses this at length in Federalist 78.