Help a noob levelling their arcane trickster

Hey there. I'm a new DnD player doing a milestone Strahd campaign. Currently playing as a level 4 arcane trickster rogue, but wondering if it's worth multi-classing.

We rolled our stats and i got really lucky with 20 dex / 18 int / 12 con so don't really need the ASIs I think.

I was thinking going into mage for the next levels and taking blade singer just because it sounded cool tbh, but would it be any good or am I better just sticking to straight rogue? I like the idea of getting multi attack but it seems like I'd need to invest quite a few levels into mage to get that, and maybe just improving my sneak attack is better?