You can go way higher on max-nodes and max-areas nowadays if you got some RAM. That generates smaller gmapsupp files, because less tiles means less duplicate data in overlap areas
Below is what I do
For Splitting
java -Xmx13000m -Xms4096m -jar ./splitter/splitter.jar --max-areas=9999 --keep-complete=true --precomp-sea=./preload/sea.zip --max-nodes=2500000 --polygon-file=./preload/europe_large.poly --max-threads=4 --resolution=15 --output-dir=./splitmap/ europe-latest.osm.pbf
-Xmx13000m
Make more RAM available, assuming you have. Here 13GB, assuming you got16GB in your system or more
--max-areas=9999
Way faster splitting with less passes. Again, only works with more RAM
--max-nodes=2500000
Less tiles means less overlap areas, and smaller gmappsupp files. 2.5mio works just fine with the mkgmap default style, and even more if OP wants to drop things
-polygon-file=./preload/europe_large.poly
Optionally. Drop this option if you don’t want to convert a custom area and it will convert the entire pbf file
For final map generation
java -Xmx13000m -Xms4096m -jar ./mkgmap/mkgmap.jar --style-file=style_lowpoi --gmapsupp --process-destination --process-exits --housenumbers --remove-ovm-work-files --max-jobs=4 --latin1 --index --poi-address --check-roundabouts --check-roundabout-flares --family-name=OpenStreetMap --area-name=DACH --route --add-pois-to-lines --poi-address --link-pois-to-ways --add-pois-to-areas --bounds=./preload/bounds.zip --precomp-sea=./preload/sea.zip ./splitmap/*osm.pbf
--style-file=style_lowpoi
This points to a custom folder with style. In mine I drop many many POI I do not care about
--link-pois-to-ways
We really want that option, it makes sure that something barriers or gates blocking the way are used in routing
--bounds=./preload/bounds.zip
--housenumbers
We also want that bounds.zip file from the mkgmap website and housenumbers, otherwise the address search is not working on the device