try

HullAccumulator

Creates convex or concave hulls for groups of features. One hull feature is output for each unique combination of values of the attributes specified in the Group By parameter.

This looks like a cartographic generalization problem.

You can also give AreaAmalgamator with GroupBy enabled on an unique attribute a try.

See which one (HullAccumulator or AreaAmalgamator) gives the best results for you.

Here's a different strategy that is helpful in some cases:

• Bufferer with a generous amount n (be sure to use the cap style that best conserves the original shape)
• DonutHoleExtractor to get rid of any holes and just keep the outer shell
• Bufferer with a negative n, meaning a negative buffer identical to the buffer introduced in the first step. Make sure to use the same cap style as in the first step.

This should give you a result somewhat similar to the HullAccumulator, but in my experience it sometimes gives you more control over the result.