I assumed it was but apparently upstream capacity is 50% or so of downstream. I may be wrong so anyone is welcome to correct me if I am.

However even if is 50% I expect thats still plentiful as long as downstream isnt saturated.

How many 'hundreds of GB' of data are you talking ?

After your initial backup of all systems, incremental backups won't be significant (or shouldn't be).

I can see where you're coming from, but suspect there are significantly lower charges associated with traffic now, else every smaller ISP would be getting their service from LLU services.

{If someone can provide links to where Openreach explains its traffic charges, that'd be useful, too. I have seen the cost for the line/service for Fibre and have assumed that with BT Broadband having scrapped the '300 GB per month' FUP, there has been a significant reduction in cost, or perhaps complete scrapping of traffic charges, because of the potential data throughput available on fibre, at any speed.}

I am currently doing the same thing with Backblaze, and have my entire catalogue of FLAC music to backup....