Hidden and read-only columns should not appear in any activity, it’s not the best practice to force saving data on them as they are intend for an internal usage only.
Working on such fields might cause your list to be unstable and corrupted, so you need to be extra careful when working on them.
For on premise version of SPARK Workflow:
for O365 version:
For the content type: We have allowed updating the built-in hidden column “ContentTypeId”, like this
You need to set the GUID value of the content type, and you can get it like this:
List settings >> select the content type >> copy content type ID from the link parameter (ctype)
Use this activity(Set column Value/ update Item) to update content type field by ID