They look pretty happy to me.
Bear in mind, I'm not saying I'm a pro, but I've had them on and off for years, different individuals.
Your light looks ok, although I might alternate between the upper and the lower shelf.
Understand that these are rainforest epeiphytes, not from hot desert 'cactus' conditions. Humidity is nice and may explain why they seem thin to you. Humidity can be helped with a humidifier, or natural summer humidity depending on your location ( but not with pebble trays or misting). If possible to place outside in full shade for summer into fall, that is my preference. I live where summers are warm and humid. With protection from rain in September/October, I can give them shorter days and a dry rest then.
Currently have one truncata purchased this past winter, still in original pot, with nice plump 'leaves' on windowsill, and a huge gifted one and a small one potted in terra cotta outside in full shade, looking healthy enough but not very plump as we've had a very dry month.. They all tend to hang except newly purchased, which I think stand up because they just came from greenhouse..
Did they bloom this past season for you?
I've seen one live ( but not bloom well if at all) on a poorly lit kitchen shelf for five year still in the original (shrinking when dry peat) potting and getting water trickled over every week or two.
My understanding is, truncata would have spiky serrations, buckleyii would be smooth (er?) like yours and so would Easter cactus ( Hatiora garnerii?)
