@kimberly3ct
So let me ask a clarifying question and make some comments, and see where that takes us.
When was your spouse's 65th birthday? Medicare is only retroactive 6 months if his 65th birthday was more than 6 months ago. If you are at or near his birthday, it will only be retroactive to the 1st day of the month he turned 65.
HSA contributions can be made any time during the year. It is not the date of the contribution that counts, but the overall total. If he is covered by a family HDHP and medicare is really going to be effective 9/1/25, then his contribution limit for 2025 is $6366. It doesn't matter when the contributions were made--they could even be made after enrollment--as long as the total for the year did not exceed $6366. If his total for the year is more than $6366 (and you correctly determined his medicare start date) then that is the amount you will need to remove.
If you are covered by his HDHP, you are also eligible to contribute to an HSA in your own name. You don't have to have an employer, you can open an account at any bank that offers HSA (shop around because they will usually charge a monthly service fee if not sponsored by an employer). You could contribute up to the combined family maximum (contributions for 2025 can be made up through 4/15/2026) and you can continue to contribute to your account after he goes on Medicare as long as you are covered by a family or self HDHP. For 2025, if he contributes $6366, you could contribute $3850. (I can explain that calculation if you need me to.)
