You'd need to learn both "th" sounds properly (eg. as in "think" and "that"). Trying to replace them with another sound like "d" or "ch" is simply a mistake - you'll either say the wrong word, or at least say the word incorrectly.
I agree that "sadat" and "sad thacha" make no sense. I wouldn't understand what you'd said either.
Perhaps an audio example would be clearer? Have you tried saying the phrase on vocaroo.com? Then we could hear you.