Why not amend 2021  and do correct depreciation in 2022 ? 

I know amending is an option. I'm just trying to figure out if filing Form 3115 is also an option. 
I've been reading about the 2-year rule and not sure if can only file 3115 if missed 2 or more years or can use it for any missed depreciation (e.g. previous year only)

I run the other way from form 3115

How much to depreciate, how many months in 2021?  A California mansion or an Alabama trailer?  Is the amount missed even four figures?  Is the tax consequence that much?  Is this the only mistake on the return?  Sometimes "do no harm" means "let it go."

If 2022 has not been filed, no, Form 3115 does not apply.

Not so bad once you have it figured out 🙂