I actually wouldn't be surprised if they announce it to investors either immediately before or during the fiscal year earnings release. They have to make projections for the next FY, and absent of any new hardware, they're going to be projecting large drops in pretty much every metric. That's a tough sell to investors if your only commentary is that you're in "uncharted territory". I could see them releasing a press release along the lines of
the original 3DS announcement, simply confirming they're going to release a new hardware platform this fiscal year, and it will be backwards compatible with Switch software, possibly not even including the name of the new device. Enough to head off concerns about declining sales of the original Switch while leaving the actual reveal for a later date.
I honestly don't think there's a big deal with announcing it before Zelda releases. Last of Us Part II released literally a week after Sony's not-E3 PS5 showcase in June 2020 and still
broke sales records. I don't think they'll show, or talk about, higher res version of ToTK running on the new console until after the game launches, but simply announcing that a new console exists shouldn't meaningfully impact software sales.