I did a search first and found few helpful mentions.
Is it possible to see the interior? I read on the turismoroma website that you need to go through coopculture for tickets. That website didn't recognize the name or the Italian equivalent.
If you can see it, is it worth a visit?
Not sure about the interior but you can combine a visit with the Protestant Cemetery (where John Keats is buried) and climb from there to the orange grove and rose garden. Great view of Ciccus Maximus and across Rome from the orange grove. Or you could visit the Testaccio market, which is pretty amazing.
I believe that the interior was previously only open to visit a couple days a month, and only with an accompanied tour. Nothing about it on the coopculture website anymore, You might try contacting the Special Superintendence of Archaeology, Fine Arts and Landscape of Rome per the phone # on their website page referencing the pyramid?
https://www.soprintendenzaspecialeroma.it/schede/piramide-di-caio-cestio_3006/
