Aren't you breaching OpenStreetMap (OSM) Copyright?

NO, and neither are any of the other map producers that are using OSM data, including GARMIN, Outdoor Active, Thunderforest, etc. (who all sell maps and services based on OSM data).

There are over a hundred companies using OSM data and selling maps and services that use their data (quite legally).

From OSM’s own site:

“You are free to copy, distribute, transmit and adapt our data, as long as you credit OpenStreetMap and its contributors.” and “You can publish/sell maps, databases and services which combine OSM's data and other datasets, including commercially available ones.”

I modify the data extensively, and the way the data is shown, and add extra functionality and features for off-road specific activities.

The resulting maps are Copyrighted by me as I add extra data that I personally created to them.

More details can be found here: https://support.talkytoaster.me.uk/portal/en/kb/articles/if-the-raw-osm-map-data-is-free-why-are-you-charging-for-the-tt-1-50k-look-n-feel-maps