I propose to change "sub anniversary" to "sub milestone" (or something similar) since it is being used for a number of months one has subscribed instead of marking the annual recurrence of date when one subscribed. It can sometimes be confusing, especially if a streamer speaks about it and forgets to override the meaning of "anniversary" by stating that it is a monthly one.
The wording becomes even less accurate if in some months in between one has not subscribed.