Before anything real gets engraved, the wood gets interviewed. This attempt is a grid of the same mark burned into acacia offcuts at stepped power and speed, then oiled, to see what survives finishing.
The finding: acacia's dark heartwood streaks engrave nearly invisible at the settings that look right on the pale sapwood. One coaster can hold both in a single face, so a fixed setting is a coin toss.
The working rule out of this grid — read each blank first, and put the mark where the pale wood is, or go deep enough that the streak can't hide it. Slower, but the grain was always going to win that argument.