If you can repair the reactor, then it stands to reason you have the basic understanding of some of its principles of operation... meaning, SF would have had enough to start R&D on its own version by the time VOY got back, and have a prototype about 10 to 20 years later... and the first versions could probably be large ones used on starbases by say early 25th century... but beyond that point, reducing the size of the reactors would be possible to start using them on starships as means of replacing dilithium and antimatter (possibly late 25th century or early 26th century).
I think though that it would take some degree of time, even under optimal circumstances, to learn how to use a foreign/new technology with some degree of success, if at all. For example, I was recently rewatching "Balance of Terror" and "The Enterprise Incident," and the latter is about the
Enterprise successfully capturing a cloak. Scotty manages to jury rig it to cloak the ship, but only barely, and the FASA Romulan books mention that Starfleet captured several other cloaking devices but weren't able to achieve the same results, and some such missions were hazardous because Romulan ships would rig the cloaks with sabotage devices (explosives typically) if they were removed from the original vessel. In one case, a Starfleet ship managed to cloak but then wasn't able to uncloak or be detected on sensors, so the crew was trapped and apparently lost.
To use some non-Trek examples, Battletech has a lot of technological build options for vehicles but every advanced system typically has a trade off, if mainly expense and weight. If you want to add more weapons or gear to a unit, you might choose to install an extralight (XL) fusion engine to save space, which provides the same output as a standard engine of that type. The main downside is that the weight is saved by downgrading the basic shielding on the engine (it's much thinner and weighs less), so it's both more expensive to install and much more vulnerable to critical hits or damage. It can be a useful balance, but some pilots don't like the increased risk involved if they get into a losing battle.
There's also a substance like endo steel, which was a variant for internal structures that required manufacture in an orbital, zero-G environment. Such an environment allowed for several metals to be alloyed together with essentially perfect bonds, with the resulting structure being able to support more mass for equipment than a standard IS. But the tradeoff was that it was bulkier and cost more internal space as well, and such advanced factories became prime targets when the Succession Wars broke out. Endo steel became a lost technology for a time.
Gauss rifles, which fire projectiles magnetically (similar to a rail gun), have been popular weapons on heavy units for centuries because they have excellent striking power at range and generate very little heat. But they're also very bulky, with the standard model weighing 16 tons, just shy of the lowest mech weight class (20 tons). This means they're traditionally more restricted to massive vehicles that have enough space to carry them, and that lighter units don't have the option of employing them. There are a number of Gauss variants that allow more flexibility, but they're still considered at the heavy end of the spectrum. Also their ammunition loads are relatively limited per ton, and a critical hit can damage their magnetic firing system and disable the rifle.
There are also real life examples of technological advantages that weren't adopted regularly for many years, for practical reasons. Muzzle-loading rifles were known to have range and accuracy advantages over smoothbores for a long time, but didn't become regular issue for years because the rifling had the disadvantage of fouling the barrel even quicker than a normal barrel. So a rifleman had to make his shots count because he would likely get fewer of them at a time. Likewise, the use of camouflage patterns in places like North America to help blend in (as opposed to colorful national uniforms) was used by some soldiers and frontiersman, but increased the potential risk of being fired on by friendly troops because you weren't wearing the traditional colors.
All that being said, though, I do agree with your point that the Feds could have looked into other options.
I've seen it suggested in some sources that the Romulans are one of the few powers to use heavy plasma weaponry, due to it being more destructive but less stable than comparable systems like phasers or disruptors. And although I think the Federation ban on developing cloaks is fairly silly in its limited canon context, I could see where Starfleet wouldn't necessarily see the need to use them as regularly because of the high power cost involved.
