Just start the backup and wait 3 months. That’s not that bad. Are you losing access to the data soon? If not, just let your auto sync take care of it piece by piece.
I had an emergency situation where I needed to move the data and upload was 25mbps. Stupid cable companies not understanding why people need symmetry. It would take approximately a year of continuous upload and I didn’t have that. So I use 4 Starlinks, aggregated them to the cloud server with a VPN and now I had 225Mbps to 425Mbps upload. It took about 3 weeks but all the data was moved.
Not just unaffordable, just not available.
That house where I needed to transfer data was in a neighbourhood only had copper phone lines from the 1960’s for DSL and then coax cable. The maximum possible was 400Mbps down and 25Mbps up. Over the years it was increased to 800Mbps down, still 25Mbps down. I paid over $1000 USD a month for that shit internet. Because the only alternative was 4Mbps to 8Mbps upload.
This is a major metro area, 700k people. Starlink was a game changer. Not symmetric, but waaaay better.
There’s only so much bandwidth on the cable line and they’ve spent ages marketing download speeds as the measure. If they go from 400/25 to 400/50, 99.9999% of people wouldn’t understand and wouldn’t pay extra. But make it 425/25 and people will buy. Bigger number more better.