Map for garmin etrex 10

I am trying to improve on the gmapbmap.img in the Etrex-10.
Using osmfilter and mkgmap I can make a nice map with the larger towns and cities but adding national boundaries does increase the file size enormously.
For example
osmfilter germany-latest.o5m --keep=“place=city or ( place=town and population>=20000 )” >de-cities-only.o5m
osmfilter germany-latest.o5m --keep-nodes= --keep=“all boundary=administrative admin_level=2” >de-boundaries-only.o5m
osmfilter germany-latest.o5m --keep=“natural=coastline” >de-coast-only.o5m
osmconvert de-cities-only.o5m de-boundaries-only.o5m de-coast-only.o5m -o=de.osm
java -jar /usr/share/mkgmap/mkgmap.jar --remove-short-arcs de.osm

gives a 29 Mb de-boundaries-only file and a 1.3 Mb 6324001.img. As I want a gmapbmap for nl, de, cs and at combined this exceeds the capacity of the Etrex-10.

I would like the boundaries and coastlines to be more chunky and by that save a lot of space. I have tried a few things like --reduce-point-density but lack of understanding of it all makes that I have not yet been successful.

All help and explaining greatly apreciated,

I dont know if it all make sense. How much internal memory does the Etrex 10 have?
Here you can find some other lightweight osm garmin maps:
Maybe it is better to download the boundaries from another source:
Load it all into Josm and save it as osm.

I do not know too. I think that a map with large cities and boundaries only is practically useless, but that user has another point of view! Had he solved his problem?

Garmin eTrex 10 is equipped less then 9MB of internal memory, practically I do not advise to load maps more than 5 - 6 MB: first of all, to reserve some storage capacity for tracks and POIs; secondly, the larger map is rendered very slow. But even this minor memory is fully enough to store a map of a whole region, available for a few days hike, i.e. over 90% ))) of users.

But such small size requires to delete all unnecessary details from the maps, as Andrzej Popowski do, But the requirements of different users will be different! My friend was charmed with Popowski’s Fenix maps, but wants to have mount peaks, shown on the map. He was lazy to learn all tricks of command line program mkgmap, so he simply used to delete all unnecessary objects from the map the full graphic editor MapEdit++ - a free version of GPSMapEdit by Konstantin Galichsky.