I think thats a lame excuse just so Nemesis would work
Lets compare the two situations:
(
Note: I've seen very little of Voyager but did see the finale.)
Lets start with Voyager and its weapons/defenses.
You propose that the Federation would keep and publicly use the weapons and defenses Voyager used in the finale in a very public way.
1/ Violates the Temporal Prime Directive and Temporal Accords.
2/ They
could keep it secret if they chose as a relatively limited number of Federation personel knew (did any Borg survive with the knowledge?)
3/ It provokes other races to use time travel to gain military advantage and the Klingon ships at the very least are capable of it (as shown in STIV).
So to use these weapons violates their own laws and provokes other races to do the same.
The Cloak Tech in Enterprise:
1/ 4 Separate races were known to have Cloaking tech. (holoship, Suliban, Romulans and Sphere Builders)
2/ At least the Vulcans and Klingons outside of Earth were aware of it.
3/ You insist that the Romulans would have abandoned it and therefore not used it in the Earth - Romulan war. No actual evidence for that abandonment just your insistence.
4/ No explanation why Earth/Federation didn't have a "how to fight invisible ships 101" as a "theoretical" course to prepare their Captains in case the Cloak resurfaced in the future.
To suppress the knowledge among their own people leaves not just the races using cloaks an advantage but those other races who have seen cloaks (and the Suliban were widely active and presumably used the cloak just as widely).
So please explain why Earth/Federation would suppress knowledge among their own race that was already known widely including to at least 3 hostile races (assuming the sphere builders can be assumed never to return). Those races being Suliban, Romulans and Klingons. (Likely the Andorians too knew of them from the Spheres in the Expanse).
Now explain why the Federation would violate the Temporal Accords and provoke other races to do the same risking an all out time war.
Final Note: As to the Temporal Accords and Temporal Prime Directives the only episodes I have seen where they are referenced are the DS9 Tribble episode, Enterprise and the Voyager finale. So I may very well be missing "nuances" in understanding how they worked.