The cisterns of Theodosius are not as popular as the Cisterns of the Basilica, but I recommend them to visit, because it is not crowded (at the moment - October 2024), there is a wonderful light performance that reflects the history of the city. A nice bonus: the entrance ticket costs 650 lire (cheaper than the Basilica Cisterns).
I recommend coming here first, and then to the Cisterns of the Basilica, so you can get into the story better.