To add a few thoughts:
You first need to decide if you are comfortable with filing Jointly or not. You started off saying you wanted to keep things separate due to your husband's legal problems, so maybe filing as Married Filing Separately would be best for that aspect, even if it results in more tax. You can discuss it with a tax professional and/or lawyer to determine if your husband's legal issues may be a consideration or not.
Although in many cases filing Jointly results in less tax than filing Separately, that is not always the case. This COULD be one of those cases where separately might work out better (at least for some of the years).
Despite popular opinion, if you and your husband file Separately, his Social Security is NOT fully taxable. If he has $0 of other income, only 42.5% is potentially subject to tax. But his Standard Deduction would eliminate any tax.
If you filed Jointly, MORE of his Social Security could be subject to tax due to your income. While you would benefit from the Joint Standard Deduction, the increased amount of his taxable Social Security could result in more tax. Based on the amounts you mentioned for your income, most years would still probably be a better tax-result filing jointly, but I suspect that 2020 MIGHT end up better filing Separately.
While the other comments are correct that a person can't claim their spouse as a "dependent" when filing Separately, there *IS* a rule that when a spouse has $0 of gross income, the other spouse can claim his "exemption" when filing Separately. It is possible that you misunderstood and that was what the tax preparer was referring to. However, because some of your husband's Social Security is potentially subject to tax (which your tax preparer may not have realized), that rule does not apply to you.
I agree that going to a good tax professional is your best option. You can first discuss if you are comfortable with filing Jointly due to your husband's legal problems. Then *IF* you and your husband are comfortable with filing Jointly, your tax preparer can determine the best tax-result (as I mentioned before, 2020 in particular could possibly be tax-beneficial to file Separately). And your tax professional can help you determine the pros and cons to how many tax returns to amend (if any).
