when they told your Thai girlfriend so, it might be possible to open the account when you go together. There is no law preventing them to open bankaccounts, just bank policy.
I also think less is more, especially when you take your time at your home country.
When they need more info, they will ask.
Collect everything that is not asked in the first place only because some embassies asked it as extra info from some applicants, might also work against you, as it is seen as a red flag.
I also don't see problems when they ask for 500K, and as long as they don't need it in a Thai account, there should be no problem, most easy would be just show at immigration online you have the funds.
immigration might like funds in a Thai bank account, which seems logical for a non-o visa, but for DTV which is not meant for people living continuous in Thailand, that does not make sense for just 180 days.
It should not be too difficult to check foreign bank statements