I can theoretically hold off on returning Tenant A's security deposit until after the end of the month, but I don't know if that serves a real point.A few small points - these tenants are basically kids (pretty new renters).

As soon as a tenant gives notice, I copy the template and create a new checklist, assign all the tasks (in your case they would all be assigned to you unless you have an assistant), and then set deadlines for particular steps like "Tenant Turns in Keys" or "Refund Deposit." 

This is unless you are allowing your buyers to go under contract and conduct full diligence which I can't imagine you are allowing since it would put your deposit money at risk, correct?

Is that 5k left to pay on top of their deposit amount?

As a seller I would counter your “certain to close argument” with “great, I want a $20k, or higher non-refundable earnest deposit.”

I'm not sure if Baselane has a limit but I have one account per building + security deposits so would need at least 10. - The ability to create online debit cards has helped a couple of times but I'd categorize this as a nice to have. - Free would be nice but if it's paid and has extra functionality I would consider. - Interest earning (not a ****** 0.2% either).

Look at the section called:"What do I do if my landlord does not give me back my security deposit?"

We gave the builder a 25K deposit - but they've only gone out there one day and didn't break ground.  

When I'm on vacation, grilling is often part of the plan, and having to pay a propane deposit and a $50 cleaning fee could be a deterrent.

It's easy enough for me to use myself and has a lot of other features that help me manage my rental portfolio and take more of the day-to-day management in house, like vetting tenants, collecting security deposits and rents and its other banking features.