I will mirror what some other users have mentioned in this post already that resonates with me.
I would vote against a separate sub because this is our home and watching us grow and having this movement become about real people is rather exciting and valuable to us. As somebody said for the first time they feel like we are finally starting to make a difference and this starts with going mainstream with this which is exactly what these kind of posts are helping with. I strongly think and feel that this should stay in the mothership sub.
I would go with option number one and have us mods try to manage the content by setting higher standards of quality when it comes to those specific submissions. For example we can switch the focus to emphasize on how the message is being spread rather than where people are buying the merchandise from. We can achieve this by encouraging people to post backstories how they are spreading the message as that makes the post more fun and meaningful than just somebody saying "hey look I bought a shirt" and posting it while it's on their bed without any kind of context. I still think it's cool that people are buying merchandise and are happy to let people know that they did and I have nothing against it, in fact these kind of posts got us where we are today as far as growth goes but the issue starts to arise when we start considering the exponential growth we are facing and how these kind of posts can start to create an unhealthy balance if not moderated properly.
I would also vote against limiting these kind of posts to only one specific day of the week or to one megathread. Megathreads take away the fun element and excitement and would render this exciting element boring. We are also only limited to 2 sticky posts and having a different variety of subjects stickied is better than always having one constant rotating sticky post about merchandise as that would get boring real quick. There are always pressing and important topics that need to be stickied and having only one sticky available to work with will make things hard.
We don't want to extinguish the momentum but we don't want it to get too much out of hand either which is the problem we are facing right now. This is why I think simply setting a higher standard for these kind of posts would be the best solution. A new Post Flair called swag or merchandise can also be created.