Florence Car Rental - SIXT at Central Train Station

There are not ZTL restrictions on Sunday, but remember that the lane along the former cloister of Santa Maria Novella, now a Carabinieri station, a tourist information center, and also the back of the church apse, on the opposite side of the station front, is reserved to buses and that does not expire on Sunday. I have got a fine on Christmas morning driving along the cloister (my good, late father told: go that way, it is shorter).

Sixt's office is still in the station building, so I'm pretty sure the return location is still the underground multilevel garage under the Piazza della Stazione.

Assuming Piazza Taddeo Gaddi as your preceding departure point the directions are below:
https://maps.app.goo.gl/XqrzqtGkSdkb7LVi7

As lachera mentioned on Sundays the ZTL is not active, however the bus lanes are always active, so make sure you don't travel onto a bus lane.

The image below shows the beginning of Via Jacopo da Diacceto, where you will turn to go toward the station. As you can see the light rail travels on the right lane of it. Do not drive on top of the track path, because you will get a ticket, Sunday or not.
https://maps.app.goo.gl/uNWMuTFBQKzYTGtG9

Actually use the above StreetView to see the itinerary all the way to the station. The entrance to the underground garage is as soon as you enter the Piazza della Stazione on the right side (next to the TIM Store). Note that the cars all stay in the lane that doesn't have the tracks, the tracks are for the light rail trolleys only. Stay out of the tracks' path, follow the other cars.

Thank you both for the great directions! Much appreciated!