I've noticed that a lot of the na-adjectives I know (which admittedly is not that many) seem to be of Chinese origin and a lot of i-adjectives are kunyomi. Is there actually anything to this, etymologically speaking, or is it just a coincidence and I need to learn more adjectives?
Actually, while it's true that い-adjectives (ク活用 and シク活用) are all (are there exceptions that aren't modern inventions?) native Japanese words, it's not true that な-adjectives are mostly from Chinese. There are huge classes of words that that conjugate as ナリ活用 and are of Japanese origin. タリ活用, on the other hand, are nearly entirely words from 漢語.
Most native な-adjectives follow certain self-similar patterns:
- ～か：静か 遥か のどか 仄か 僅か 密か 定か
- ～らか：麗らか 清らか 平らか 安らか
- ～やか：円やか 軽やか 健やか
- ～よか：膨よか 健よか
- 新た 可笑し 大き 稀
And タリ活用 are mostly literary expressions like 堂々たる ～然とする and so on.