I have another potential idea.
Why don't we remove its triggerable status, and just have sustained burn be a thing ships do whenever they're doing a sustained burn?
That is, whenever a fleet has been flying in one direction for long enough, they get an extra burn point. Each additional burn point above the maximum 'nominal' burn would decrease acceleration/deceleration, and increase the amount of time flying in one direction necessary to get more sustained burn bonus.
Small changes in direction will drop a couple of points of this (presumably handled in-engine as a much larger amount of granular points, say, 1000s of sustained burn points that build up over time and drop when you change course) but probably just the very top end. Large changes will fairly quickly dump all of your excess speed.
This makes maneuvering a little bit more of a skillful thing you can do, would work fine with the AI as it's something that just -happens- to every fleet (though would need to be simulated off-camera instead of actually tracking fleet direction), and it'd naturally lend itself to travel.
If you tune it right you don't actually need to set a maximum speed (though you could) - ships will just get faster and faster on reaally really long trips - but will almost certainly never get to silly speeds because each extra burn point will take, say, twice as long as the one before, and at some point they're going to need to maneuver, and at that much above your nominal burn speed (which could indeed be called your maneuvering speed) you'll drop lots of excess burn with only tiny adjustments.
Replace the sustained burn ability with a much less often used "Drop to maneuvering speed" ability, which just initiates an immediate deceleration to maneuvering speed, and you're done.