Tiling issue with multiple maps

Hello,
I am trying to compile the whole of Britain’s OSM data using mkgmap for my Garmin etrex 30.
I want to create two layers, one containing points (to route to), which goes underneath with drawpriority 1, and one that renders streets but not points which goes on top with drawpriority 1.

I should stress that I can get this to work with just a small area perfectly, and it renders and routes exactly as I want. I set the mapname for the underneath routing layer to 53240001, so that the filenames don’t clash and overwrite each other. With just a small area *.osm file, it just generates 53240001.img and 63240001.img and these on their own work fine - they don’t seem to need a gmapsupp.img.

The problems seem to start when I have to use the tile splitter to generate multiple *.osm.pbf files. Now the *.img files still get generated - the routing layer then goes 53240001.img, 53240002.img, 53240003.img, etc etc. and the top render layer goes 63240001.img, 63240002.img, 63240003.img, etc (the default names - I don’t specify the top layer’s mapname). And if I just have the tile for my local area, which is now 53240028.img and 63240028.img, then all is fine. But if I have them all on, then some don’t display, and the routing doesn’t work for them either.

I did manage to get them all to display when I used mkgmap to create a gmapsupp by combining the 5324****.img and 6324****.img files.
However, my garmin did then fail to route to somewhere in a different tile (first it just said ‘route calculation error’), and thereafter it turned off whenever I tried to do an ‘on road’ route.

Any idea what I could be doing wrong or ideas to try?

For the underneath routing layer I’m using mkgmap options as follows:

mapname=53240001
series-name=OsmRoute
description=OSM Route
product-id=43
country-name=United Kingdom
country-abbr=UK
index
route
drive-on-left
report-undefined-nodes
style-file=mkgmapstyles
style=route
region-name=UK
region-abbr=UK
draw-priority=1
road-name-pois=0x6400

and for the top render layer:

series-name=OsmClear
description=OSM Clear
product-id=42
country-name=United Kingdom
country-abbr=UK
index
route
drive-on-left
report-undefined-nodes
style-file=mkgmapstyles
style=clear
region-name=UK
region-abbr=UK
draw-priority=2

and then to combine them, just mkgmap -jar mkgmap.jar --gmapsupp *.img.

It seems to create a file called ‘osmmap.img’ and also an ‘osmmap.mdx’.
What are these, and will my garmin need them?
Could it be an issue that the ‘osmmap.img’ from the compilation of the tiles got included in the *.img wildcard of the combination to the gmapsupp?

Thanks for any help

Also, when I create a gmapsupp.img from other already-compiled img files, does it contain the data within them, or does it merely ‘reference’ them, i.e. do I also need those files as well on the garmin?!?! I presume the latter due to its file size, but just thought to check!

if it references them, can it be made not to - i.e. to gather all the data within itself - maybe this would solve my problem?

A gmapsupp.img contains all tiles so you dont need the other img files.
I wonder why you make the pois layer routable, afaik two active routable layers can break the routing, and besides that, only lines are routable anyway so better make only the streets layer routable.