How much of a language's vocabulary should be loanwords (both directly and derived through SC)? I know there's no direct answer, but I'd like to hear a good range, as well as anything I should take into consideration.

Generally, if your language is the big empire (for instance Chinese) it will contain few loanwords. If your language is isolated it will contain few loanwords. If your language is subordinate to a big empire (for instance anything that borders China) it will contain a sizable portion of loanwords. If your language is in close contact with some other language(s) it will contain a sizable portion of loanwords. Statistically.

tl;dr between 2%~70% traceable loanwords everything goes (there are very likely several languages outside this range but I suspect most fall in it), just pick something and figure out where you get your loans from.

Also welcome back, and don't be so melodramatic about everything.

Additionally remember that in an expansionist and/or colonial empire loanwords are likely to sneak in from the territories.  Languages also seem to have their own character in how easily they accept loanwords, from Mandarin's [2% loanwords] "native compounds can describe the concept better than your barbarian original" to English's [75%] "give us all your vocabulary in used notes and don't even think about pressing that alarm"+Yes, these characters tend to stem in a large part from identifiable historical, cultural and even grammatical/phonetic concerns.  So perhaps you want to decide on a characteristic attitude to borrowing (or more than one, through different time periods), then make up reasons to justify it.

Pro-loanword factors:
- Close prolonged contact with another language
- Existence of a non-native prestige language
- Phonotactic adaptability [? a particularly weak one I think]
- Existing accepted history of loaning

Anti-loanword factors:
- Lack of sustained contact with other languages
- Cultural resistance at a sufficiently grass-roots or enforced level (sorry Académie française, you don't cut it)
- Language is a prestige language in its broad cultural sphere
- Difficult to adapt potential loans to existing phonetic/morphological patterns [weak]

A factor that needs to be considered together with the other factors mentioned is whether technolgy and culture are heavily influenced by speakers of other languages. All other things being equal, the more technology and cultural items are taken over from speakers of other languages, the bigger the likelihood of loanwords.