@joonaspessi kindly did some research on the PostGIS implementation of ST_Simplify(), which is not implemented as a thin wrapper around GEOS:

Actually started the implementation of ST_Simplify but noticed that the PostGIS has different behavior for collapsed geometries and collection types in comparison to GEOS implementation.

Achieving full PostGIS compatibility (including all the params) would probably require custom Rust implementation.

• Both PostGIS and Sedona-DB uses now GEOS for ST_SimplifyPreserveTopology
  • https://github.com/postgis/postgis/blob/master/postgis/lwgeom_geos.c#L935 - Entry point
  • https://github.com/postgis/postgis/blob/master/postgis/lwgeom_geos.c#L966 - GEOS function call
• PostGIS uses custom implementation for ST_Simplify
  • https://github.com/postgis/postgis/blob/master/postgis/lwgeom_functions_analytic.c#L58 - Entry point
  • https://github.com/postgis/postgis/blob/master/liblwgeom/lwgeom.c#L1851 - Core custom implementation

I wonder if the geo implementation of Simplify (if it exists) already ticks some of these boxes...since it is a more involved function to implement, it is probably worth going straight for a custom or geo-generic-alg based approach.