ZSH only displaying last 16 or so commands with history. HISTSIZE & SAVEHIST are 500

history 1 (or history 1 -1) will display all entries from 1 (first entry) to -1 (last entry). You can also use history 1 100 to display the first 100 entries or history -100 -1 to display the last 100 etc.

For more information see the section on fc in man zshbuiltins. (history is equivalent to fc -l).