The real problem with derivative characters is that of a catch-22. Unless if writers knew what to really discriminate between those characters and the original, the characters are still derivative to the point of being redundant. Mike Smith pointed out the problem with Supergirl is that she’s very redundant.
The very same problem can be applied to Superboy. The more writers try to connect him to Superman very strong, the more redundant and erratic he gets. Writers can’t even decide what personality is he supposed to have. Is he a flirty extroverted young lad? Is he a misguided, emotional clone of Superman? Is he a prodigy? Is he a replacement for someone else? Is he stuck in another form?
He’s practically been through this characterisations in the comics I’ve read and in some cartoons I watched. In other words, the more writers connect him to Superman the less they actually have anything to do with him in the long run starting with an established personality.
In Barry Allen’s case, that would be easy. If someone were to make him interesting, we’d have to address the fundamental problem with his characterisation. As a scientist, he’s not that dedicated to this work as he prefers to fight crime face to face.
He makes much better sense as a detective given his preference for investigation and that he could also make an excellent escape artist even if that means having to tone down his powers to make way for a believable characterisation.
As for Bart Allen, if he can’t be related to Barry Allen his connection to the latter could be explained by a bootstrap/predestination paradox via time travel when Barry accidentally replicated his backstory onto another person. That person would become Phantom Lad, Bart’s new persona.
One could go the extra mile and make him come from Europe, with darkness powers to boot. That’s something writers don’t bother doing now that he’s not related to Barry. And sometimes people do have coincidentally the same or similar names.
Writers actually tried to do something radical with Supergirl before, starting in the late 1980s when she can’t be related to Superman she might as well be related to Lana Lang, had very different powers and was made into an angel. It’s not straightforward when this involved fusing with another woman but that’s a hell lot more inventive than what Superboy and Bart are getting these days.
It’s not easy but it’s not impossible it’s just a matter of learning both prior mistakes and new influences to know what to do with them. Like making Barry into a detective, Bart an Irishman with darkness powers and Conner being Jimmy Olsen’s brother of sorts. It could be doable and could salvage these characters for good.