I was just reading this post from last year and had this follow-up question https://www.reddit.com/r/AskHistorians/comments/4j76ir/why_did_it_take_so_long_to_discover_australia/
Not to discourage answers but, at least regarding China's apparent lack of interest in colonization, there is this previous thread with links to other threads (by /u/Searocksandtrees) that might have the answers you're looking.
HTHH