What's the difference between the "-ism" and the "-istry" suffixes?

Solution 1:

-istry should not be interpreted as a suffix itself, as it's both -ist and -ery.

-ism

1 : a distinctive doctrine, cause, or theory

2 : an oppressive and especially discriminatory attitude or belief

-ist

a suffix of nouns, often corresponding to verbs ending in -ize or nouns ending in -ism, that denote a person who practices or is concerned with something, or holds certain principles, doctrines, etc

-ery

a suffix of nouns denoting occupation, business, calling or condition, place or establishment, goods or products, things collectively, qualities, actions, etc.

To answer your question, the distinction would be that -ism would denote a system, or in the given "Terrorism" denotes some ideology. And -istry correspondingly an occupation.