Off the top of my head, this kind of murder is solved by a combination of surveillance footage, cellphone evidence, forensics such as blood or unique tire tracks, eyewitnesses, and DNA.
I'm thinking of recent cases such as the Idaho 4, the Gilgo beach killer, Alex Murdaugh.
The known surveillance video has not led anywhere. IMO it's too late for them to get more.
I'm sure they've done a geo-fence search, there wouldn't likely be too many cell towers/phones roaming that isolated area at that hour.
So, likely the perp didn't have their cellphone on. (Does that mean the driver at the gun store didn't have their phone on - or are LE limited in their ability to get a warrant for that, potentially innocent behaviour?)
If LE have probable cause on someone, they may still be able search that person's phones, though as time passes, phones are changed out.
Evidently, there was limited forensics - Missy was not moved, the weapon wasn't left at the scene, the killer escaped into rainy pavement, etc. If the entry in the FBI database is correct, and there was a gun, that could be matched.
There were no eyewitnesses. So the only chance is if someone saw the killer in that costume at some point, or killer confessed to someone.
Given the time frame the killer was gone from home, the FBI profilers I've listened to, commenting on similar cases, note the person likely lived alone (or no one was home that night), so they didn't have to explain where they'd been at that hour. They were unlikely to have to go to work the next day. Perhaps they worked a late shift. That would imply an isolated person (fits with the behaviour of role playing).
Because of the costume, for sure no DNA.
So I don't agree it's laziness or incompetence. IMO, someone in the local/state/FBI police forces would be as keen to solve this as the many, many people who have posted on this thread and the internet generally (every crime youtuber has analyzed this case). Many detectives would want to solve this, not just for their own satisfaction, but it would be huge boost to their career.
JMO