Unless the ship firing phasers projects an elongated beam of a warp field around the phaser beam (a beam inside a beam) to be able to fire them outside the warp field at a target at Warp that's a fair distance away but still within say 300,000 km - that can help explain how ships can fire energy weapons usually meant for STL combat in FTL... at least it helps explain a few instances where ships weren't close enough to merge Warp fields.
If you plan on firing & hitting ships that are traveling at Warp that are anywhere within the upper range of 3 km - 300,000 km long distance range, that sounds like it would require a "Special set of dedicated Warp Coils" wrapped around the muzzle of the beam emitter to even do that, otherwise the beam should veer off like crazy.
It's definitely a "Zany Idea" or "Wonder Weapon" if there ever was one for the 24th century.
You've seen my justification for the "Close Enough" w/o merging Warp Bubbles, only merging Warp Fields; but the other reason the Nebula Class was probably able to fire at warp besides being close enough is probably their "Navigational Deflector" + it's own Warp Field should provide a Assymetrical Shaped Space around it that allows firing directly ahead w/o Warp Field distortions affecting it. Given how short range the "Navigational Deflectors" are and how powerful they are, it makes sense that there's a small extended frustum of space ahead of your vessel that you can control via positioning to shoot at each other if you get "Close Enough". Something both the pursuer & evader controls by controlling the distance between the two. We all saw how close the Nebula-class was too the Prometheus-class when it fired it's phasers.
At any other greater distances, the beams would either get affected by the spatial distortions from the Warp Drives or not be affected, but still have to deal with time/space lag even if you're shooting out of the broad-sides of the warp field/bubble where it's relatively distortion free.
Once they are within range, Warp fields merge, and you can use phasers (this seems to be the most likely explanation).
I concur, but at that distance, you're definitely in knife fight territory since you can easily make out the details on the others hull by looking through a window with your natural eyes.
Also, on the notion of evading phasers at Warp... generally that doesn't happen because course corrections at warp are not recommended and could fracture the hull.
Usually, ships tend to execute minor and careful course corrections while at Warp, but otherwise tend to travel in more or less a straight line.
Different Warp Nacelle types offer different amounts of FTL Manueverability, there's a reason why StarFleet went with Paired Parallel Warp Nacelles instead of Warp Rings.
On average, the Warp Ring configuration was 17% more Energy Consumption Efficient than Twin/Multi Warp Nacelle designs
The Coleopteric Warp Drives was Notice-ably more Energy efficient, but MUCH less manueverable at High Warp speeds.
StarFleet abandoned the design becaues they needed the flexibility of manueverability at High Warp speeds.
I can definitely see the "Coleopteric Warp Drives" dominating the commercial FTL landscape due to it's Natural "Energy Efficiency", 17% more Energy Efficiency over Paired Parallel Warp Nacelles is "Too Good" on a economic/resource efficiency level.
But generally, similar to going fast while flying in the air, your course corrections are over very broad distances, it isn't "Sudden Movements" like you would see with a Newtonian Focused 3D Thrust Vectoring Space Superiority Fighter in STL maneuvers.