Heroes did!
Just like that weird "call me by name call me immaculate one plz" thing that makes Nopes' Sothis exchange with Flayn completely nonsensical, the "Rhea became berserk because she despaired and felt like Sothis would never return" felt like a quick fix-it made with adhesive tape and the contents of a gummy bear.
Like, this doesn't explain why Rhea doesn't become red in VW.
Just going into it, yeah, I 100% agree the whole despair thing is mostly just a handwave to justify why she doesn't go berserk in VW.
Yet another reason VW arguably shouldn't have existed, it causes too many problems by being Silver Snow with a layer of paint over it.
And you won't make me believe that she wasn't despairing the hell out of it after witnessing Zanado and being its lone survivor, nor after seeing whatever remained of Sothis after Nemesis deboned her.
We can make a point about her being more "angry" than desperate, same as in CF when Seteth'n'Flayn kick the bucket - even if her mental state's pretty low and precarious at this point - but then, VW still stands, as in, Rhea realises after eating missiles and "zumba'ing for 5 years in Enbarr's finest 5 stars resort" that her Mother never gave a flying fuck about the remaining Nabateans, or her role as the Goddess, and yet, she doesn't become red.
But in the interest of playing devil's advocate, I think you basically answered your own question, and in a funny way it reminds me of Tales of Berseria. The villains needed Velvet to be overcome by anger and despair, but the issue is that while she had plenty of the former, she was so driven and furious that it blocked her from giving in to despair despite how horrific her circumstances were.
In a similar way, when Rhea had a goal to set her mind toward, be it killing Nemesis, killing Byleth, or resurrecting Sothis, she didn't give in to despair. In VW she's in mostly the same boat as with SS, but she has Nemesis's return to focus on and keep from despairing.
Silver Snow is the only point where she has no enemy to fight and has to come to terms with the fact that her mother is dead, that she chose to stay dead, and that she's never coming back. It's honestly understandable that she finally broke after all the strain she's been under.
I think it's a bit unfair to say Sothis "never gave a flying fuck about the remaining Nabateans, or her role as the Goddess". She's dead. Her time has passed. Incarnating herself and becoming the goddess again would require her to take control of Byleth's body, which I think most of us would regard a horrifically selfish act.
“A blasphemy, you mean! How could the goddess of order violate the most fundamental natural law?”
Reviving the dead is the mark of villainy in this franchise... at least until Engage had Alear die and get back up three or so times, the last two being in consecutive chapters.