It's very tricky to answer this sort of cross-periodic question since conquests and rule historically have looked very different and have happened for many, many different reasons and in many different contexts.

We might need to unpack this a bit. If you look at Greater Iran, the modern state is a relatively small part of that. The part of it that held and holds most of the population (basically, the mountainous border regions in the west, southwest, north and northeast) is an even smaller part. Historical maps can be very misleading - not only do they tend to conflate areas where the amount of political control vary widely, the ones you see tend to be the maps of an empire at its greatest extent.

Your premise that mountaineous regions are harder to rule directly over is probably broadly correct, but I think this is reflected to a degree in the history of Iran. Consider that the Achaemenid Empire itself is famous for its establishment of satrapies, that could act semi-autonomously, as an administrative innovation. It took the Parthians something like 120 years to conquer all of modern Iran. Something like the Timurid Empire at its height might look impressive, but the relationship between ruler and the ruled was largely a tributary one. The difficulties of the Abbasid caliphate in establishing direct control of many Zoroastrian communities is well-known; it took nearly 200 years to establish Islamic rule in Tabaristan and Parthia (ruled by e.g. the Zoroastrian Bavandid and Karen dynasties). Sometimes these rulers paid tribute, sometimes they refused. Most of the history of the region in the Islamic age up until the establishment of the Safavid Empire is a messy one of dynasties rising and falling, invaders (Seljuks, Mongols, etc) conquering and fragmenting, and so on. Even after Safavid rule, you had numerous rebellions and crises, several resulting in dynastic changes even if there was largely administrative continuity.

But consider again the aforementioned areas - Fars, the Zagros mountain range, Tabaristan - these regions did not historically have very many major population centers since there aren't that many spots suitable for establishing cities, so the key areas you need to control are fewer than one might think. Ruling the coastal areas in the north and south was obviously very desireable for reasons of commerce, and ruling the rest of the Zagros mountains becomes largely necessary not least for territorial continuity between these two regions given the harsh climate of the east-southeast. There has also been, largely, cultural, religious and linguistic similarities between the groups that have inhabited these regions, which of course makes ruling them together easier.

So I think once you consider the details of demographics, geography, rebellions and fragmentations, as well as what rule looked like in practice (semi-autonomous regions, tributary relationships, etc) the region both appears a lot less unified historically, and what degree of unification one still observes looks a lot less strange.

For sources, there aren't really any good "grand history of Iran"-type works (nor do I think it's a particularly fruitful approach). The Encyclopedia Iranica is always worth checking out as a first stop on any one topic. If you want recommendations on any specific period I can help, but I'm relatively shaky on the Parthian era and the 800-1200 AD period, and not all eras have particularly accessible works written about them.