In Bangla, Hosonto (U+09CD) is used to create a ligature, which joins adjacent letters. For example ক্ক is created using ক + ্ + ক. But sometimes we need a non-joining Hosonto (ক্ক). To make it possible, traditionally we use a Zero-width non-joiner (U+200C).
The problem with ZWNJ is that, when the line is too long and line wrapping occurs, the word is broken into two lines. To keep the word as a whole, I need a character, something like “Zero-width non-breaking non-joiner”. But I don’t see such character in Unicode. So I think, Word Joiner (U+2060) is the best option.
To me, Word Joiner sounds like “joins two words”. But in my case, I need to join two parts of a single word. So, the question is, is it correct to use Word Joiner here?
U+200C ZERO WIDTH NON-JOINER has no effect on line breaking. Its absence or presence does not change where line wrapping can occur. If inserting a ZWNJ within a word causes that word to be broken across lines, then whatever application you are using to view your text does not implement the standard correctly.
ZWNJ is the only correct character for your purposes. More than that, using U+2060 WORD JOINER could in fact lead to inconsistent results. Much like ZWNJ does not affect line breaks, WJ is not supposed to affect joining behaviour (it is defined as “transparent” in that regard). While the standard doesn’t explicitly mention cases like this to the best of my knowledge, one could reasonably argue that inserting a WJ between the two letters in your example should not change the way they are displayed.