Will moving my 400K savings to a fixed account affect my visa extension eligibility in Thailand?

For Bangkok Bank accounts this will not work. When they print the statement showing the account activity and balance, they deduct the amount used as credit card collateral. So if the account has 400K in it and that amount is designated as credit card collateral, the statement will show a balance of 0 baht. Immigration doesn't care that the balance in your bank book is 400K. They go by the printed bank statement. So before you try to do it this way you need to check with K Bank to see if they do it the same way.

Unless I'm mistaken the one that is tied to a credit card via a fixed account will not work

Keep in mind there's no obligation you keep the 400K baht in a thai bank account after you get your yearly extension inked in. You can move it as long as you get the balance back up to 400K baht 2 months before your next yearly extension

If the fixed account has instant access