Would it make any significant difference if I do the form 3115 next tax year rather than doing an extension for this year with it?
It could. But considering it's been wrong for over 10 years, I don't think it will be that much of a difference to matter really. But it does depend on the numbers. Overall, since the 3115 is not simple and really needs to be done by a professional, you may find it difficult (if not impossible) to find an experienced CPA this late in the tax filing season. It might be more realistic to tackle it next year with the 2023 taxes, which allows you time to set up things with a CPA to get it done quick and early in the 2023 filing season for next year.
Secondly, why amending with 1040x specifying the change just once and recalculating the correct and remaining depreciation for the rest 15.5 years won’t work so that we have to do form 3115?
Because by not depreciating the property, you have used what the IRS calls an "impermissible method". When using an impermissible method for depreciation for 2 or more consecutive years, the IRS requires the 3115 to make it right.
This is convered in IRS Publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf starting on page 13 under "Change your accounting method". One of the examples shown where the 3115 is required is, "A change in the treatment of an asset from nondepreciable to depreciable or vice versa." So based on what it says in the pub, you are required to fix this with the form 3115.
