Well even if you are not ready to apply, you can fill out the form to discover what documents they ask for.
There's no template for a sponsor letter. You need to add a sponsor letter only if the 500K is in a bank account in your name. If it's the case, just write something along the lines of "I will will pay for all my spouse's expenses during his/her stays in the kingdom, and he/she has no intention to work in Thailand".
I'm not saying this will always happen to everybody. I'm just telling you that you won't be able to say "ok, prove me that it's assessable income". You will have to prove that it's not.
And yes of course, the regulation is new, so we may not have examples yet, but it could happen, and if it happens, you won't be able to tell the Revenue Service "prove me that or that". Because in the end, the burden of proof is on you.