honestly if youre asking about wether shining is important or not at this point, i wouldnt even start talking about multishining. Its such a nuanced tech for advanced play, its not worth attempting to implement yet. Start with the fundamentals.
my recconmendation is to start out using shine for simple shield pressure. things like nair > shine are very common in competive play. practice jumping out of it to get follow ups like shine > nair, shine > uair, shine > dair, shine > fair, shine > side b, etc. also practice using it to turn yourself around in the air if you want to follow up with a bair for an edge guard or something. when youve got simple applications like this developped, then you can get a bit fancier.
the next important tech skill past simple applications would be wavedashing out of shine, imo. Its very essential to properly punishing variable DI, and is your quickest, most mobile escape route out of your shine. It also opens the door for additional shield pressure by allowing you to bait out, avoid, and punish hasty OOS maneuvers. on many characters at lower percents, shine > WD > shine > WD > shine etc is combo.
past that, id even say shine > JC grab is more important than multishining at anything but advanced levels of play. its easier to do, and is more rewarding, though not as frame tight.