A Chromebook could work if you'll have wifi/cellular internet when trip planing?
I've never really got tablets (I do a lot of typing and usually want a real keyboard) but I've plaid around using Chrome OS, hillmap* and a 62s to do this.
If you go this way you need an internet connection to view maps on the chromebook as there but you can load gpx files to/from the garmin either by plugging it in with a usb cable and dragging the gpx file to the right place in the filesystem.
You can't use all the full on garmin desktop apps (they require windows or os x) to do things like upload new free topo map file gpsfiledepot so you'll need to do that somewhere with a real computer.
Advantage is that chromebooks are cheap small, light and available with cellular internet. I've thought about using one to write trip reports etc while on a long road trip.
*Full disclosure, I wrote hillmap so of course I use/recommend it. Other web based mapping sites like caltopo, gmaps4 etc should work too.