How about ذ (zal), ز (zaa) and ظ (sor)? Perhaps they are more 'z' than 's'? Then there is ث (tha) though that is more of a 'th' sound I think, but people in the subcontinent pronounce it as an 's'. The differences are really subtle to me.
It's way dialectical, of course, but in MSA at least 'ز' is a [z] sound, 'ث' is a [θ] (as in "three"), while 'ذ' and 'ظ' are variants of [ð] (as in "the")— with ظ being the pharyngealized version, similar to 'د' and 'ض'. The differences are much easier to hear than to reproduce :P
The pronunciations you've listed are used in some Arabic dialects. In classical Arabic, ذ is pronounced as <i>dhel</i>, and ظ is more of a <i>dhah</i>.
I'm a native arabic speaker too. I think I misunderstood your intention of using "dh" in "dhad", I would have used "thad." This misunderstanding stems from the fact that we're both trying to write a sound that cannot be expressed correctly in latin.
