Need to push backups across two WANs to maximize off-site backup upload speed

Solution 1:

The biggest factor in your scenario is your backup application. If your application makes a single connection (i.e. a single TCP flow) to the destination, there's little you can do to speed things up. Any attempt to "load balance" across multiple connections is likely to cause out of order packets and slow things down.

The good news is (based on a quick calculation) that with one 150Mb stream, you can upload all your data (ideally) in about 15 hours.

If you application supports multiple connections, it may be possible to spread them out over multiple links, but it will require extra hardware/software on both sides. If you can live with 15 hours, that would seem to be the way to go.

One more thing: do not use WiFi for your backup: WiFi is half duplex, and you will struggle to get 150M throughput.