I ditched the plan of sending my backups to the cloud. I wrote an old school script to send it via FTP and my speed was around 10Mbit/s, the upper limit of my contract. Could update it to 40Mbit/s, but even then 10GB would need 35min to send.
That's why I ordered a small USB stick (Samsung FIT Plus 64GB). Benchmarks showed 60MB/s (= 480Mbit/s), leading to 3 minutes for 10GB of data. Once per week I will move the data to my external hard drive and clear the stick. I think that's a viable approach.
Afterwards I tried to refurbish an old notebook. An old two core 32bit CPU with 2GB RAM, so no need to trash it, I will give it away, maybe to a school or a youth center. But first I had to check if it is too slow. I installed Debian without a desktop environment and added i3. Will do further investigations, e.g. how well Libre Office and a small browser will perform. If this won't run, it's probably too slow. That would be sad, because the notebook has been well maintained.