The whole covid thing is screwing these guys hard. Last season I waived my local diners last payment, it was a $2k payment.Restaurants and bars have been the worst affected by COVID and somewhat understandable why they're late paying you. They're more than likely paying suppliers that are crucial to the day to day business and puts you to the back of the buss.
If they've been a good customer and you've had them for a while I'd suggest you try to work with them to get current.
To watch them go from being busy all the time to closed doors without having any fault or say in the matter is hard to watch, they employ some great people.
I agree a conversation needs to take place and the tone of that conversation on both sides will warrant whether or not it needs to be litigious. I doubt it, you already suggested they are "slow pay" not "no pay" and they have been clients awhile.