First, I’m sorry about losing dad

I just lost mine, and I know there’s a lot of emotional upheaval (honestly, a mix of loss and relief) as we chart a path forward.
Second, I’m very glad you’ve got the basic legal stuff done. That makes everything easier.
Third, do you have siblings that you are working with and/or trying to negotiate with? I am very fortunate in the sense that so far, my three siblings and I have agreed on everything related to Mom’s ongoing care and how to disperse Dad’s various assets. As executor you make the decisions, yet as an ongoing member of the family you also still have to interact with other members of that family. Are there any disagreements or points of contention brewing? If so, that’s perhaps where to focus your attention, get some solid options, and see if concensus can be reached on those points.
From what you’ve said so far, I don’t see any downside to closing your parents’ cards. Your dad certainly won’t need his anymore, your mom probably shouldn’t have credit anymore. The only complication I can see is if there are indirect financial implications for closing her accounts and having her credit rating slip. One example I can think of is that her homeowner’s insurance, auto insurance and/or any adjustable loans she may still have, might get more expensive. But any/all of those may not apply.
If you have siblings which are already making a fuss over how to proceed, it might be a good idea to bring in a mediator to nip that in the bud and/or clarify what rights you have as executor to make decisions even over sibling objections. That probably also varies by state. Hopefully that’s not an issue, but if is it I wouldn’t waste time arguing with siblings. I’d cut to the chase, clarify the legal rights of each, and just move forward. Time/money spent now working to clarify everyone’s options, will make the decisions easier to make.
I hope your decisions come relatively easily and that when the dust settles, you’re satisfied with having done the best you could do with any given situation.