On the one hand, the most "important" thing for you to do right now is to just keep on proceeding through the starchart. As long as you aren't struggling with enemies, you're good to just keep on going with that without taking too much time to farm unless you want to.

That said, it never hurts to get things early. Multishot really is a very powerful stat, and so long as you have enough Endo and Credits to rank it up it will be a big boost to your overall damage output. Spy missions can also get you pieces for the Ivara warframe, who is useful for a lot of stealth-oriented situations.

HOWEVER, please note that the rifle multishot mod doesn't drop from spy missions - the shotgun multishot mod does. It looks like the easiest (albeit not the fastest) way to get Split Chamber from the planets you've currently unlocked is to do invasions and side with the Corpus enough times that the Grineer send an assassin killsquad after you, then wait for them to show up and kill them when they attack you. Just be careful - if they win, you'll take a long-term debuff unless you spend some rare materials to remove it. Details on the assassins here.