The advantage of the "this vs. thin" example is that they are pretty much identical in structure. They both start with the "th" group, followed by the same vowe

The advantage of the "this vs. thin" example is that they are pretty much identical in structure. They both start with the "th" group, followed by the same vowel "i", and finally end in a consonant even if it's a different one. None of the usual "but it's grouped with this", "but it's followed or preceded by a vowel or a consonant", etc. should make a difference. The "ch" is pretty much always read "tʃ" instead of individual letters. And I think "ci" is also pretty much always pronounced as "si" (Pacific, specific, city, scarcity, etc.). As an individual letter yes it changes but you can find a semblance of a rule when using the ch vs. ci examples. Fun read: Ghoti is a creative respelling of the word fish https://en.wikipedia.org/wiki/Ghoti https://en.wikipedia.org/wiki/Ghoti