This sounds like misinformation. You should always conduct due diligence before joining any bank, but there are several online banks that are better than larger physical banks.

Ally savings has a 3.75% interest rate right now. They also have a no penalty CD with 4.35% I think. It was 4.75% a few days ago.

You can change your W-4 to single to increase your withholding. Doesn’t matter if you are married or not. This is basically just a way to tell your employer what to withhold.

When filing taxes you will still go with married, filing jointly (or married filing separately in rare cases).