You grab anything you can, really. From Advent events, you should buy all the pots, tickets and 1 copy of the Eidolon - and then whatever you can. The more copies of the Eidolon and SSR weapon you can grab, the better. Can you grab some SR weapons too? Cool. Soul Points for more Souls? Cool. Eventually, you'll start being able to do Ultimate and LMBing both the SSRs won't be a dream anymore.
Your main source of stuff as a new player will probably be Raid events, which throw a lot of various goodies at you. Hopefully that won't change in the next raid, since they're redoing the system a bit.The store SR weapons have really, really attack sadly. They're around 1300 atk, whileas event SRs are 1500-1800. Disaster Raid SRs are about 1500, some as high as 1700.Each one has two HP weapons and one Assault weapon.
In the long run? You'll want to specialize into one element and have LMB 10 SR+ Assault weapons for it. A couple of HP weapons is fine too.
As a new player? You have no idea what Gacha will give you, so you don't know what element you'll go. Having a varied, even if weaker, weapon grid will cover you a lot of bases.
That said, have you rerolled into SSRs? There's a rerolling thread explaining how that works. It's a huge boost to your early game team, and it might just decide your element for you right off the bat.Right now? If you're in a Union which is active, you should be doing Standard Lilims to help your Union get Grails. Buuuut... chances are your Union doesn't have much. Active Unions have fed their weaker members almost all rewards from three categories by now. You can't be fed the fourth "personal PP" category.
Outside of events, spending your AP on daily dungeons (limit break mats), raids (limit break mats and SR weapons) and storyline (Jewels, mats for Store) is probably a good idea.
And... I don't understand your "advent union struggle." What?