Prior to setting up an airtime plan?

As you've discovered, you can import .gpx and .kml into the map page on the web site. Note that you won't actually be able to do the import until you've established an account, which involves signing up for a plan.

I've never seen the space limitation stated quite that way. I've never had anywhere near 500 waypoints on the device. I have had more than 5 routes. Although I've never systematically investigated it, I do believe that imported tracks will be "thinned" on import. Other than reducing the point count in the route, this usually does no harm. However, for certain topologies, you may find that this produces unacceptable changes in the route. The moral of the story is to check the route itself before using it in the field.

Note that the limitation applies to the device, not the map page itself. You can individually control whether a particular waypoint or route will actually by sync-d onto the device. So you can have a very large collection of waypoints and routes on the map page. But you are limited to a reasonably sized subset on the device itself. You can adjust the subset at any time, but each change requires a wired sync.

As far as forums go, this is it. The old DeLorme forums had more traffic. However, they shut down late in December. Anyone attempting to access them is redirected here. So this is as good as it gets. Many of the old hands from the DeLorme forums did make the transition, so the expertise is out there.

Awhile back I exported some PN60 tracks as GPX and tried to upload them the my explore site. Thinning the thousands of track points down to less than 500 doesn't always work well. Some of them imported OK, others were nearly unrecognizable. Unfortunately there doesn't seem to be anyway to control how they get thinned short of doing manually yourself before you upload them.