1. I'd drop the thunder sword in favour of an assault raid SR to get the assault bonus.
2. Eventually they get overshadowed by the SSRs. Vine gets replaced by Hraeslvegr, Anzu gets replaced by Sleipnir etc. The ones you wanna keep are Vine, Anzu... Annnnnnnd that's about it. I think.
3. Hmm. I'm guessing you don't cash or reroll, considering how jumbled your line up looks. I don't exactly know the meta for these cases, but I'd rather not drop Osiris for Amaterasu so you can drop Cybele for Ithaqua. Reason being Osiris has a burst gauge skill, which you'd lose for what would be effectively a 5% increase in atk and def break... not really the trade off I'd look for.
4. The meta for D'Art now is ignore the assault relic and take the sniper shot boost gun. The only one I remember is the sniper shot value, which is 30% B frame atk and def break. If possible, I'd rather you hold on to your regalias until you have a better defined team, more souls to work with... and definitely until the other weapons come out.