Are there any services that add a layer on top of google/apple/... services for encryption? Something like put a veracrypt/whatever container in google drive and use another open source web as entrance or bundle these services (google/apple/...) as encrypted ceph cluster which means every company only owns part of the encrypted data.
Another approach, host your data in full disk encryption (LUKS/...) in open source services (NextCloud/... you could find awesome self-hosted services on github) and put the password into a RAM USB with UPS. It could be sealed forever by turning off the power of RAM USB. The only problem here is that there is no commercial RAM USB.