Dont you become a Thai tax resident if you stay more than 6 month ? They could easily announce that Thai tax receipts must be shown when entering on DTV
Thais quite frequently change the entry requirements . They could easily change the 6 months DTV entry to 30 day entries, like they are doing with the 60 day visa exempt or require you to have paid tax in Thailand
UK bank account 5% interest rates , which is 40 000 Baht a year if you keep that 800 000 Baht in a home bank account , Visa agents fees are less than that