If you joined a union in the middle of a union event, you'd get all the rewards where they already met the quota. That's a lot of stuff if they're already done. Of course finding a union willing to invite someone just to have them hoard rewards without contributing might be difficult, if anything, you might be able to join a less active one and at least get something. Also you need to join a union to complete certain dailies, specifically the donating gems to union dailies that appear sometime.
Getting all the weapons of the same element is a goal to keep in mind, but it's not something you're going to be doing in a single day let alone a week. Til then, for your sub weapons, you're best off using anything strong you can get a hold of, especially with your current numbers. You do want your main weapon to be the same element as your teams focus so your soul will be the same element, so you'll be stuck using that R sword on it for your dark team until you hopefully get the next event's SSR sword. You'll slowly swap out the rest of the weapons better weapons as you progress from events and hopefully convert them all to a single element.