there’s no way to activate pro right now, and afaik that option is not coming.

Since beta 25 it is now possible (and the developer had mentioned before that it was coming).

Edit: I stand corrected (should have read your comment more attentively), what’s available is, as you say, the option to remove ads with a one-time payment, as well as the sync ultra subscription. I’m not sure how the ad removal option is different from the pro mode in sync for reddit, though.