Maybe I recall that the reason for your return at Termini is that the via Sardegna locations show as closed Saturday afternoon. I find that, even if the offices for starting rentals are closed then, the garages for Hertz and Avis returns there are staffed, in Avis's case until 4.30.
About getting a cab, if you have a smartphone there's the it.taxi app, which is supposed to work like Uber to get a regular taxi, with a GPS to your location and payment to a credit card on file, but the time I tried it, after a via Sardegna return, it didn't work. With any cell phone, I don't know if there's a language barrier, but there are the central numbers 06-3570 and 06-5551. What finally worked for us (those two had nothing available) was the number 060609, which does voice recognition of your address if you can say it clearly enough in Italian. Now I see that they also have an app: ChiamaTaxi 060609. I mention these since, in either garage, the attendant who checks in your car may not be equipped to call a cab.