Is my DTV visa still valid if I quit or get fired from my job?

When you have no job anymore you do not comply anymore to the visa requirements. As long as nobody checks this there is no problem. Because DTV is heavily abused, there might be more checks in the future, but for now only very few are checked at the border.

Just make sure to always boarder bounce rather than extend within Thailand, seems to be less scrutiny but doesn’t guarantee they won’t ask questions

I’m just doing a border bounce, much easier and less hassle.

Technically you are staying on a visa which no longer suits your purposes, but you're probably good until someone starts asking.

Likely never but there is always a small possibility that you need to prepare for.

Your visa states "employment prohibited". The Thai authorities permit online work, but I wouldn't go around advertising you're working in Thailand as that can lead to misunderstandings and problems.

You'll only need to prove what you're doing if you go in for an extension, but you can always use an alternative such as a soft power activity if you're no longer employed.

I'd imagine a work contract you update and having the 500K in your account for at least 1 month would suffice though; you can easily update a work contract yourself even for a company you no longer work for.

Since we're talking work based abroad, Thai authorities really have no way of verifying anything.

Nobody will know unless you're foolish enough to attempt to do an extension at immigration, like so many are doing.

How will they know? If you were to apply for an extension inside Thailand at an immigration office, you would need an updated employment letter from the company. However, if you do a border bounce, ie re-entry, they are not checking this level of detail.

no one will really care

Common sense....