your visa validity date starts counting down the day it is issued. You have until Nov 13th 2026 to enter Thailand. Every entry will give you a 1 year permitted to stay stamp or until the date your health insurance expires. With planning you can get almost two years out of this visa.
As a Thai tax resident any UK dividend income is paid out tax free. Find yourself a decent brokerage, I use low fee Degiro, and pick funds or shares based on your risk tolerance that have historically paid decent dividends. Aim to generate stable passive dividend income to supplement your pension. Avoid anything that has a platform or fund management fees.