I am not familiar with your Garmin or with your other computer resources. So, not sure if this would help. I loaded these maps into my Garmin but I have a different Garmin and I use some vintage software on my computer.
Europe - OpenFietsMap

Instead of the giant file, I loaded the individual smaller files (all but one are still big at over a gig) lower on the page.

I have not used these yet, I can't say if they can be used with routing software or not.