The problem with any electromagnetic coils is mechanical stress caused by stress flexing and heat as magnetic flux builds up and declines rapidly within them. That's why transformers, electric motors, etc. eventually fail.
Semiconductors, such as silicon chips fail due to an unpleasant phenomena refered to, in electronics engineering, as electron pathway degredation, which is similar to water flowing through limestone. Because of this internal resistances gradually decline at an exponential rate. A silicon device, in constant or regular use, has an expected reliable lifespan of 20 years. The faster a device is run, the shorter its lifespan.
An unfortunate fact of physics.
I would figure that it would be easier to swap warp nacelles and recondition them rather like the process the RAF ran with figthers during WW2. The average Spitfire or Hurricane actually had 3 engines, one in use, on ready to use and one being serviced.
When I used to drag race motorcycles, back in the late 1980's, I use three engines in a similar rotation pattern.
I would imagine that the UEN would adopt a reliable, though not the best performing warp engine design, mass produce it then fit them to whatever vessels they required to have warp drive.
According to TOS the final and decisive battle of the Romulan War is fought during 2162 over the moon Cheron, which orbits the planet Neptune in our solar system. Perhaps the Romulans were attempting to establish a base there, perhaps the UEN had a base there, who knows?? Anyway the Romulan's offensive capability is destroyed in total and a peace is negotiated.
Clearly the UEN would not need to have a completely warp driven fleet to fight within 8 light hours distance from Earth.
A vessel travelling at warp 2 would cover the distance in 2 hours.
Another issue is that for every day in space a crew requires food, water, toilet paper, air and heat to survive. A large ship requires a large crew to maintain it, which requires a large supply hold, which requires a large ship. On top of that a large ship requires a bigger engine an fuel tanks andf another vicious circle begins.
A ship can only be as large as the engine technology, for sublight travel, can reasonably propel. An unmanned ship, such as an interstellar probe, could be larger as it doesn't have the crew and stores problems imposed on it. It only has to carry its sensor, communication equipment, computer control system, fuel and engines.
A manned ship has all of that plus the life support system and storage hold which'll have the flight duration x crew amount of food, water and toilet paper. Then there is the extra mass added by cabins and work areas.
If weapons systems are added, the mass increases further.
Short range ships, intended for space voyages of less than a month could therefore afford to be larger with a larger offensive capability.
Long range ships would be smaller and have small crews, less weapons and maybe even skip carrying a shuttlecraft.
If the UEN was intending to convert any sublight ships to warp drive then the smaller ships would offer the best options in peace time.