Being Southwest, go ahead and buy. You can always rebook at a lower fare and hold the difference for a year from now. Unless you never fly Southwest that is the best strategy.
Using the Low fare Calendar it looks like the lowest fare for a Sunday is $146. Your fare is already being pushed up by previous sales (Spring Breakers?) so is unlikely to drop much further.
Weekends fit into a special Southwest bucket, different than weekday flying. Unless there is a sudden fare sale for Grand Rapids I wouldn't expect much of a decline in Sunday fares.
If you have any WN points the fare is only 13000 points. Buying with points insulates you from fare changes, since unused points come back to you.