Just sent the first 17 letters to the owners of distressed properties on my long list.

You should seek a Lender who can provide a formal Certificate of Approval Letter upfront and help Prequalify DSCR properties in advance to simplify the process. 

I recently received a letter from the tax assessor explaining tax assessed value of the property increased to the purchase price I paid for. 

Nobody wants to sell anything.We're always doing grass-roots marketing like dropping or mailing letters, networking with neighbors, and reviewing wholesaler deals.

Once I have bought the tax deed and 12 months have passed and I send the letter to the owner of the property, payment for the taxes.If owner don't pay, Do I get the sheriff to evict the person from the house, so I can take possession of the house after the  45 days have passed?

I feel like it's safe to assume that when most people see the letter in the mail it just goes straight in the trash anyway. 

Despite multiple on site visits and mailed letters he wasn’t responding.

If it's different than the property address then consider using snail mail (a letter) to contact whomever is getting the tax bill.

I have assumed their leases and added an addendum that includes a few items (no auto renewal, new payment address, etc… ) which I am planning on asking them to sign.I sent non-renewal letters about a week after closing (early April), which gives them way beyond the 30 days' notice.

I have a mailing list of over 3000 recipients and are aiming for a personal touch with letters that appear handwritten and are signed in ink.