Some other subclasses have situational Channel Divinity features, but every other subclass has at least one non-situational, bread and butter ability you will be using regularly in the situation of "combat". Both of the Oath of Devotion's Channel Divinity features are situational.
Oath of Ancients has one blatantly situational CD feature and one that is easy for high-STR or high-DEX enemies to make a save against.
Oath of Vengeance has one situationally useful CD and one that is fantastic but only used against bosses.
Oath of the Crown has one situational CD and another that's pretty mediocre in general.
Oath of Conquest, OK, both are useful pretty often.
Oath of Redemption has two incredibly situational CDs that are moreso than Devotion on their worst day.
So, no, not every other subclass has "at least one non-situational, bread and butter ability."
The other situation is setting up ambushes. Again, this isn't something you can reliably assume you can pull off, especially with the Stealth Disadvantage you probably have from heavy armor, though at least if you fail to pull it off you don't blow your Channel Divinity. But while it isn't an oath violation, there is something thematically wrong about having a signature ability of the classic paladin most reliably trigger when ambushing foes.
As several posters above have mentioned, it's not just for ambushes. The thing lasts 10 rounds. If you're in a situation where you expect combat to happen within the minute, which is pretty often, go ahead and pop it and you'll still get use out of it.
The obvious solution is to make Sacred Weapon a bonus action, and since (unless I'm mistaken), Oath of Devotion is generally considered the weakest PHB paladin subclass
You are mistaken. Redemption is the weakest, by far. And Crown is usually considered not much better; it's saved by one Oath spell from being at the very bottom. Devotion is average or slightly above average.
(Also somewhat of a tangent, I consider Ancients somewhat overrated by many. It's good, but many overlook that the aura which halves spell damage does not work against non-spell AoEs such as dragon's breath.)
Any obvious mistakes in that analysis? Any suggestions for better ways to fix the feature?
It doesn't need to be fixed.