I would hold onto Golem Vanquisher, Vorpal Reaver, Aegis Barrier and Twin Fang. The other legendaries are memey/useless in competitive play (although I can’t actually tell which Vanar spell that is, it’s so small!)
As far as game modes, Gauntlet is the best way to build a quick collection of you’re good at it, getting 3+ wins nets you more value than you spend on it. But as a new player I would expand your skills on ladder.
I’m an Abyssian main who’s played Lyonar, Abyssian and Vanar in S, feel free to add me (IGN same as here) or @ me if you need any help!