Qfyilyi

The pronunciation of words spelled with "-ti-" in German seems to be a little complicated. I read in a book that "-ti-" is pronounced as "-zi-" (I believe only when a vowel follows) in words like Aktie or Patient, but that words from French such as Aristocratie may have stressed /tiː/ instead.

It seems to me that it would be simpler to just use spellings like Akzie and Pazient instead. That kind of use of Z has precedent in some other languages, such as Italian ("paziente"). I know that "ce" and "ci" in Latin-based words are often replaced with "ze" and "zi" respectively in German, and I am wondering why the use of the letter Z did not also become usual to represent the same sound in words that have "ti" in Latin. I wonder whether the spelling "ti" has been kept out of some concern that using "zi" would obscure the etymological distinction between words that had -ci- and -ti- in Latin. Did any scholars or spelling reformers ever comment on this topic?

Yes. That is actually a significant part of one of the recent spelling reforms:

Es werden neue Varianten eingeführt: Differenzial, Potenzial, potenziell, substanziell, parallel zu den schon eingebürgerten finanziell, tendenziell;

Neuerungen der deutschen Rechtschreibreform von 1996

Whereas previously the 'correct' forms would have been Differential, Potential…

This is just an ongoing process of customs and bureaucratic negotiations, and resulting in always imperfect and inconsistent outcomes.

By the way, while now it is Aktie, in older books you may find Akzie:

Allgemeiner anzeiger und nationalzeitung der Deutschen, 1818

Background:

I'd say that there is no real rhyme nor reason behind it. Reformers as well as opponents argue with 'rules' to apply (if…then), but those rules are then inconsistent as well. So yeah, has to be memorised, Duden has to be bought in a new edition… lather, rinse, repeat in a few years. You know: a camel is just a horse designed by a commission.

A conservative critic of the reform efforts up to 1997 was Theodor Ickler, who wrote a ghastly attack on plans, reformers, contradictions, inconsistencies, idiosyncrasies of the participants, proponents, rules and results in "Die sogenannte Rechtschreibreform – ein Schildbürgerstreich" (PDF). It's a delightful read.

Given the one example from the question and the reforms applied: one change that is apparently unthinkable, despite ample prior art in that regard would be Nazion. Looks like only a 'special' group of people that is currently still seen as a minority really favours such a change. Given the supposed rules that should govern such a decision for change in orthography, there would absolutely no obstacle at all. Politically or historically however…