July 6th 1863 draft registration has him listed as 21 former service Home Guards living Rockford Caldwell Co.

Married Aug 8, 1866, to Hattie C. Smalley in Caldwell Co. She filed for a widow's pension on 9 Dec 1884 based on service with
"C 6 U.S. C.H.a."
The pension was approved app#321914, certificate#356334.

There is an index entry for him as Sargent Caldwell Co Missouri Home Guard which you mention.
-----------
There is a Thomas Kerrins Smith listed as graduating medical school University of Michigan in 1867 listing home as Mirabelle MO. The Dr dies in California in 1908. This is your guy I'm sure due to the Kerrins name (mom's maiden name).

------------------
However there is a second Thomas K Smith that is in 1880 census as a shoe and mercantile merchant with wife Harriat who dies 1911. His obit indicates he was active in Salina, Leavenworth, and St. Louis.
https://www.newspapers.com/article/the-salina-semi-weekly-journal-tk-smith/164421715/

So do we have a conflation between the two... I will work on it some more for sure.