Harri,

to keep your workflow you could possibly generate the .imgs with overlay info --transparent and give them increasingly higher --draw-priority to control, which is painted on top of which. Modern devices allow to enable and use multiple map layers this way, although heritage devices may not.

> Combine attempts with like "java -jar mkgmap/mkgmap.jar --gmapsupp basemap.img fixme.img oneway.img"
Wouldn't the mkgmap syntax treat at least the last file named on the command line as typfile?
(https://wiki.openstreetmap.org/wiki/Mkgmap/help/TYP_files)

But: In an old shellscript I do successfully merge multiple .imgs into one final .img using mkgmap with an args-file for the final map (including --gmapsupp) and several --input-file=....img lines for the individual .imgs. The individual .img's typfiles all are given individual names during the individual maps creation, although they typically (but not enforced) where just copies of the same source typfile in my usecase. No typfile named for the final map. The outcome works on Garmin Edge devices as well as QMapShack on Linux.

   # concatenate maps from subregions (if any) to main region
   ...
   # create options.arg file
   echo "overview-mapname=${MAPNAME}" > ${ARGS}
   echo "overview-mapnumber=${MFID}0000" >> ${ARGS}
   ...
   echo "draw-priority=30" >> ${ARGS}
   echo "transparent" >> ${ARGS}
   echo "gmapsupp" >> ${ARGS}
   for SUBDIR in $(ls -d "${PROJDIR}/${MREGION}/"*/)
   do
      if ! [ ${SUBDIR: -5} == ".bak/" ] 2> /dev/null
      then
         # ${SUBDIR} is subregion not .bak dir
         for IMGFILE in $(ls -d "${SUBDIR}"*.img 2> /dev/null)
         do
            echo "input-file=${IMGFILE}" >> ${ARGS}
         done
      fi
   done
   echo "output-dir=${PROJDIR}/tmp/" >> ${ARGS}
   echo "verbose" >> ${ARGS}

   # run mkgmap to tmp
   mkgmap -c ${ARGS} 
   ...