@MT61 You need professional guidance and income tax preparation.
By not claiming a depreciation deduction initially, you adopted an impermissible method of accounting. As a result, you can file a Form 3115 and make a Section 481(a) adjustment to claim the depreciation that was foregone and accumulated over the years.
See https://www.irs.gov/instructions/i3115#idm140516453720416
This form, however, and scenario should be dealt with by a tax professional.
