An Italian entertainment site called "MLD Entertainment" has posted up a new promo for the italian market discussing the success and currently planned pipeline for Generation 5 as we move past the movie. One particular paragraph gives exact months for three major 2022 releases in particular, including May for the special, September for the series, and Christmas for a mini-movie which is something we haven't heard anything about until now.
I don't know exactly where they get this information, but it fits the estimated timeline for those first two. A Christmas Special definitely isn't too far fetched for the pony franchise. Have the google translated quote:
Netflix is already working on the next contents that will be available in 2022. In particular, a 44-minute mini-movie is scheduled for May, the first season of the TV series will be launched in September, while another mini-movie is scheduled for Christmas. . All content features the new ponies.
Thanks to Andrew for the heads up!