そういえば

そ

う

い

え

ば

Meanings

now that you mention it; come to think of it; speaking of which

usually written using kana alone

Used when something said reminds you of a related topic or fact. Often introduces a new thought or question.

それはそうと
それはそうと and そう言えば both pivot to a new topic mid-conversation, but それはそうと more firmly sets aside the previous topic while そう言えば frames the new topic as something the previous remark reminded you of.
ところでN4
ところで changes the subject fairly abruptly and doesn't need to be triggered by something just said, whereas そう言えば specifically signals that the new topic was recalled because of the preceding remark.
ちなみに
ちなみに adds related background information in a slightly more formal, informative tone, while そう言えば is casual and marks a sudden recollection.
そういや
そういえば is the standard, neutral form; そういや is a casual, contracted version used in relaxed spoken conversation.

そういう
そう言う is an ordinary verb phrase meaning 'to say so/that', while そう言えば is a fixed discourse expression meaning 'now that you mention it'.

From そう (so) + 言えば (conditional form of 言う, to say), literally 'if one says so'. The expression grammaticalized into a discourse marker.