It might not be much of a speed improvement for the OP but for me it could be the solution I've been looking for. I'm getting 1.8Mbps upload speed on my VDSL line vs 12 Mbps on a phone with a Rain sim in it.
I quickly scanned through my recent archives and over the 2018/19 rugby/soccer/cricket season I took over 70k photos weighing in at 500gb. That would only take 5 days on Rain and the family could still get full use out of the VDSL line. There's probably 10-20k other photos I took over the same period that I'd also like to back up so call it a week.
I've got about ten years of photos I want to back up so it would only take a few months even if I scheduled everything between midnight and 18:00. Rain's R250 per month is sounding pretty good, for me at least.
Who you using to backup? I'm using G Suite.