Basecamp with TOPO PRO map and fenix 6x pro solar not usable

I bought TOPO PRO map for Garmin fenix 6x pro solar and tried to get map working with Basecamp but it is totally useless as loading takes forever and if you zoom or pan map during loading, it starts loading again.

And no, you can't copy map to some external USB drive because it's tied to watch and won't work elsewere. I already tried.

Why Garmin is advertising TOPO PRO map usage with Basecamp when reality is that it is impossible to use because of loading time??

Is there anything to do to speed up watch USB connection, especially uploading from watch? Map size is about 2.5GB.

Or can I load just some of maps from watch as there are quite many of those?

Is there a way to clear some cache or something?