Yes. Some countries’ embassies still issue an affidavit of income which you can use instead of monthly deposits or banked money.
US, UK, Australia and Canada (probably others) will no longer issue these. So the only option if your embassy won’t issue one is to use 800k banked money for the initial visa and first year extension and after that can switch to 65k monthly transfers.
Tax issues are outside of the scope of this group, but afaik you only are subject to tax on income brought in to Thailand.
Many countries have dual tax treaties with Thailand and that means you’re not taxed twice, but you really need to get advice from a tax expert or accountant to work out any potential liabilities.
You have to get yours first as you’ll need the visa number in order to submit hers. There is no time limit on how quickly you can apply after you have yours.