some 6tb hdd's in th cellar would be an option, thats right. maybe an second nas/synology just for that kind of data.
but my home connection is not so fast. so i have to upload the stuff back to the seedbox just to reseed or fill an request. that may takes some time.

yesterday i saw https://hubic.com/ which may be an cheap option. 10tb for 5€ thats very cheap, but maybe there are limits on the tos.