We will be arriving at Florence Santa Maria Novella train station at 10:30 a.m. on a weekday and can't drop off our luggage at our Airbnb until at least a hour later. We'd like to spend that hour sightseeing. Our Airbnb is near the Duomo Museum. Where would our closest/least expensive/safest option be to store our bags? There are 6 people, each with a regular suitcase and carry-on.
This place claims to be just a few minutes from S. M. Novella. I haven't used it.
If you Google: Luggage storage Florence you'll probably find other options, but that one seems very convenient.
I'd prefer someplace closer to our Airbnb which is on Via dell' Oriuolo. The train station has storage, but it looks like charges start at 6 euros per bag. I've only left luggage in train station lockers before that would hold 3-4 bags. I'd prefer someplace with lockers, but am not familiar with any of the many companies that seem to hold luggage in Florence. I just want to make sure they're secure and reliable.
The place I've linked to seems to be much cheaper than the price you cite, and it has two sizes of lockers. It also mentions insurance.
I think you'll mostly find luggage-storage facilities around major bus and train stations.
Via dell’Albero is just 2 minutes walk from the station.
It is a perfect opportunity to drop your bags and visit the restaurant “Il Vinaino”, one of the best cheap eats in Florence.
I used to eat there often since it is close to where I went to college, when the Business School was on Via Curtatone.