swallow, absorb, annex, engulf
吞 centers on taking something in completely and making it part of oneself: swallowing food, absorbing ideas, or annexing territory. The idea of engulfing extends this to overwhelming consumption.
吞 combines 天 (heaven/sky) and 口 (mouth), suggesting a mouth open wide like the sky to swallow something whole.
Picture a mouth (口) as vast as the sky (天) opening to swallow everything in one gulp. 吞 is to swallow, absorb, or engulf completely.
For ドン, imagine a huge mouth swallowing a donut whole with a loud 'don' sound: don -> ドン, and the donut disappears into the vast mouth.