Anyhow, change the date of your Switch everyday before you launch a game,
and save a game before midnight, close the game before it hits midnight, then change the date after it hits midnight,
then launch the game (after midnight).
The point is to always launch the game when your Switch is in a state where "date was manipulated".
(If and when your Switch changes the date naturally, the state also changes; no longer "date manipulated" state.
So to go back to "date manipulated" state, gotta manipulate the date again.
The actual date you change to doesn't matter, just need to have at least 1 change)
This way, the seed for that den (if wishing piece used) NEVER CHANGES, even if the wild area news gets updated.
[Theoretically it shouldn't change if a wishing piece wasn't used, but that wasn't what I tested, so cannot give confirmed information]
The species might change, but it'll still be shiny, retain IVs etc.
With this way, I've retained my shiny frame for months..