The lobbies are certainly less busy these days unfortunately. And I'm not sure when would be the best time to look for players. I think your best bet would be a post on Reddit, looking for players and hopefully syncing up with them. Though if there are others on this forum who are still interested, I'd love to know.
Good question, but I'm not sure, tbh. There are more people on iosgaming, but everyone on applearcade has automatic access to Osmos, so... I'd say just try one, and if you don't get much traction, try the other...?