Hey and long time no see!
Very good question!
They are quite similar, but if we look from a wider perspective 言わば is used to explain something by comparison or giving example. It is an actually conditional form of classic “to say” so it literally means “If I were to say…”. You can also think about it as an analog to English “speaking figuratively”. Often some metaphors follow It is very close to 例えば.
即ち is simple rephrasing, “in other words”. It is closer to つまり and 要するに.
She is my best friend if I were to say we are (like) sisters.
(Explaining to a friend who Son Goku from dragon ball is, by comparing him to Superman)
If I were to say Son Goku is like Japanese Superman.
(言わば is fairly formal expression, so those sentences are meant only as examples of usage)
I hope it helps.