A few ideas for when it comes time to reopen:
Require masks to be worn from the moment they open their vehicle door up arrival to the moment they close that door. upon departure. Non N95 masks protect the people around the person wearing the mask as it severely limits any moisture droplets escaping into the air. Anyone not wearing a mask or removing their mask should be banned from the barn for a period of time (more thoughts on bans below).
I think your staff would be safer if they propped doors open and the boarder was the only one handling grooming equipment and tack. Perhaps require the boarder to use a separate halter and lead.
Stall latches and crossties are easy to spray. Require boarders to stay out of the horse’s stall. except for taking the horse out and putting them back as staff must spend time in that enclosed space mucking, etc. If you can provide spray bottles of disinfectant the boarder can even spray the latch when they’re done.
Have a schedule and limit the number of boarders based on your facility. If you have a larger facility you can have more while keeping space between them. If you have multiple boarder tack rooms you can have one boarder from each tack. room at a time (yes it adds to scheduling complexity, but the earlier you can open the more necessary it will be). Consult your boarders as to how long a time slot is appropriate (an hour might be too short, two hours too long - longer slots mean fewer visits). Let everyone know the numbers may have to be adjusted as you see how well people are able to keep their distance.
If you have a boarder staying past their time ban that individual from the barn for ten days or two weeks. Do not allow any other boarder to take any of the banned boarder’s pre booked time slots during that time (perception will be favoritism if you do - and the other boarders in that slot will notice the banned boarder’s absence). Use your judgement - five minutes over might warrant a warning the first time it happens. Ban them the second time. If you require boarders to be driving off the property by the end of their time slot it will be easy to see who’s pushing the boundaries.
Boarders may not like the idea of being banned, but if you make it crystal clear up front exactly what will result in a ban, and emphasize that boarders have full control over the actions that will result in the ban there won’t be much sympathy for anyone who gets banned. Make the ban long enough to be felt by that boarder. If most boarders are coming out twice a week then a one week ban might not be long enough. Consider having progressively longer bans - one week for the first offense, two for the second, etc.
Whatever you choose to do have a plan, have a clear set of rules, have defined consequences for those who can’t abide by the rules, clearly communicate those rules and consequences to each boarder UP FRONT before they arrive for their first visit, and ENFORCE the rules. Yes, that means someone will have to check at the beginning/end of each time slot to see who is there, but if you aren’t willing to enforce the rules then don’t bother having any. Depending on how you have parking set up you could simply take a picture of the parking lot a couple of minutes into each time slot and have a record of any offenders.
One last thought - no cellphone use on the property except in emergency. People texting or whatever have a tendency to become oblivious obstacles - in the aisle, in the tack room, etc.