Rather than have "deathworld" be a flag that causes all terrain to be prefilled, why not have a setting that creates a fill/land bridges on the next N hexagons? For example, every time you claim a hexagon, it bridges to all hexes within a 3 tile radius. This makes handling biters less trivial, while still encouraging military progression.
Other options might include either a "fill (rand 2..5) hexes on claim" or having it be chance based, or also scaling on hex quality (eg claiming a legendary hex fills 2.5x more hexes than normal).
Or perhaps filling (a large number) of hex edges with a marsh terrain (non buildable, but walkable) so that regions of biters become a problem until the hex is eventually claimed and destroyed (which could actually work well with the gradual fill options above).
