I just think the writers can raise their power levels without making them completely invulnerable and impervious for dramatic purposes.
But what do you even mean by "raise their power levels?" How do you propose it would be depicted? And what purpose would it serve in narrative terms?
The thing about "power levels" in fiction is that they're totally arbitrary and rarely have any real effect on the storytelling. No matter how many power-ups you give the heroes, the villains will instantly rise to the same levels so that they can continue to pose a threat. Any pretense of a power increase is essentially superficial. Star Trek had Starfleet upgrade from photon torpedoes to quantum torpedoes, supposedly much more powerful, but in practice they were still depicted as functioning little differently from cannonballs in the Age of Sail, causing only as large an explosion as a given scene required. The Flash has learned numerous new speed tricks and fighting techniques over the years, but they still don't make him any more effective against the bad guys, since they need to keep the bad guys free until the end of an episode or a season. If anything, asserting increases in the characters' power levels just makes it more implausible that they can't defeat the villains more easily.