That's another reason why you should focus on 1 or 2 teams first, so you don't have to struggle trying to get weapons for each team. Your dark team would be the best option since a dark team can handle all content while a thunder team will struggle against wind events. You can farm disaster raids in-between events or advent events for SR assault weapons.

Leveling weapon skills shouldn't be a problem. You can easily get most to Lv10 skill with little effort from gem gacha for R weapons and extra SR defender weapons from events/disaster.