On and Off

The main sites to visit in Athens are grouped within walking distance in the Plaka / Monastiraki district, in the center of Athens in areas not accessible by bus.

If you want to visit the Acropolis, the Agora, the Parthenon, the Acropolis Museum, the Parliament, the National Garden, see the Changing of the Guard, etc. you will have to do it on foot

Only the National Archaeological Museum is located outside this area

So, in Athens, a Hop-on Hop-Off bus will have no other interest than to take you around the city via the main avenues and to make you discover possible traffic jams rather than monuments .

Because the main sites are in the central area of Athens there is insufficient room for the hop on buses to get close to the big sites. The route is as I understand around the outskirts of the central area. You are better off just wandering around the central area on foot.
Personally I am about visiting the sites I am not a detail person so I prefer to wander and explore sites that interest me.
There are lots of tour guides in Athens. These people have to complete formal education in order to get their license. I randomly searched for tour guides.
This may interest you.
https://www.tourhq.com/greece/athens-guides

The only major site you can't reach easily on foot is the National Archeological Museum, some distance north of the Acropolis. It's well worth seeing, and I'd suggest a taxi to get there and back. That said, a good guide can improve any visit, and the cost shouldn't be too much split among six people.

From my limited experience with HOHO buses (not including Athens), they can help you see some outlying areas and get an overview of a city's layout, including traffic jams. They're a terrible way to get from place to place. Hopping on and off means, too often, long waits for the next bus, or the less crowded bus after that. Not an efficient choice.

Thanks to all of your advice. I had an inkling that that might be the case. I have been to cities where HopOn HopOff made no sense while at others it gave a nice overview making planning what to do next easier. Thanks again!