Session serialized data... kinda? Dont understand the format

It's Base64-encoded, not encrypted - there's a big difference.
And it is "fully serialized", just not via the serialize() function.

session_decode() can be used to unserialize the data, but that function puts the result directly into $_SESSION instead of returning it, and that's a quite big issue ... you don't want that; you should look for another way to do what you're trying to do.
Just add a user_id INT DEFAULT NULL field to the session table and update it on login, don't look for a complicate

PHP Code:

private function _assoc_session($account_id, $session_id)
    {
        return self::$db
            ->where( 'id', $session_id )
            ->update('ci_sessions', ['account_id' => $account_id]);
    } 


d solution.

Subtle but important details like these are the reason I suggested (in the other thread) that you don't try to deal with the low-level drivers' code. You don't know how a session works internally and therefore you shouldn't be messing with it.

Alright, I just ended up doin it your way.