Sorry for so many questions but where does (r)on come from? I think I get where it came from for the most part where ista comes from. √IStā > ✶istā > OS17: ista > OS22: isθ > S3: iθ but I don't get is where -(r)on came from and how it developed. I am assuming it came from ✶-rŏ? suf. “an old agental formation”.
No need to apologise for questions! I am happy to help.
Sindarin, in its tendency of reducing final vowels and suffixes into oblivion, sometimes compensates but adding more suffixes or doubling them.
A very clear example of this can be seen by comparing Q. ando "gate" with S. annon "gate"; the Quenya word is in fact identical to its primitive form, ✶ando; however, the Sindarin annon actually derives from ✶andondo.
The case of Q. Istar (which itself is a shortened form of Istaro) vs. S. Ithron is essentially the same. The Quenya word derives from ✶ista·ro - that is, from the verb ✶ista- "to know", itself from the root √IS, and the agental suffix ✶-ro.
The Sindarin word on the other hand adds an extra suffix, ✶-nd0 (a masculine agental suffix), and thus Ithron derives from ✶ista·ro·ndo.
Also an important thing to note, regarding the derivation you showed in your question: the verb ista- "to know" (its stem/dictionary form is identical in both Sindarin and Quenya) derives from √IS·ta-, not √IS·tā-; the latter would mean "to cause (someone) knowledge", or some such, and would look the same in its stem form in the later languages, but inflect slightly differently in Quenya.
One final thing - when applying phonological developments to verbs, note that the verb stems do not have a final syllable, because in practice they must take some kind of suffix.
To give you an example, we can take a look at the root √PʰER "ready, prompt", from which we derive the Sindarin adjective fair "quick, ready, prompt" and the Sindarin verb feria- "to make ready (promptly)".
The adjective is derived using the adjectival suffixe ✶-ja, following normal phonological developments:
The verb is derived using the verbal formative suffix ✶-ja, which produces a verb stem ✶pʰerja-, which must have some other suffix(es) added to it in conjugation in order to be a proper word; thus, the last syllable of the verb stem is not word-final, and therefore some phonological developments are blocked: