Sliding time-scale.
It's what has made comics unique from almost every medium, and allowed it's characters to persist and remain popular for longer than most every other medium.
You built your universe on it, it's a fiction tool, use it, take advantage of it.
AND STOP TRYING TO EXPLAIN IT!
STOP MAKING EVENTS THAT RE-WRITE THE PREVIOUS EVENTS, AND MAKING THEM THE MAIN FOCUS OF YOUR UNIVERSE. (although I blame the fans who keep repeatedly falling for and devouring these events)
These ham-fisted events are always the problem, never the solution.
The "Golden-Age" doesn't need a time stamp, it never looked exactly like the 30-40s, it was always full of anachronistic, science, inventions, magic, look and setting, let it be a past and STOP TIME STAMPING IT to satisfy some unnecessary fanboy desire to fact check and line things up with the real world.
You can portray a vintage, pulpy, noir, diesel-punk fictional world, that conveys the past, where Batman would have operated, what maters is the visual, without putting a date on it.
It never needed a date, it never will.
That's a bunch of fanboys trying to convince themselves this is more than absolute fiction.
The same goes for the portrayal of their present and future look of the DCU.
As far as characters over-time acquired from other companies or imprints.
Integrate them, develop them into their own cities, where they are the absolute definitive champions of that series and all spin-offs.
Yet also allow for them to cross over with the other DCU characters.
Although never treat them as secondary, looking for, or awaiting approval of the other DCU characters.
If it's a main character in the DCU, and you want the readers to perceive them as such, they should exist in that main U.
And by all means have alternate universes, where fun alternate time-lines and versions of characters can exist.
That is part of them fun.
And don't put a stupid limit on those either.