Have a look at SwissTransfer. Its run by Infomaniak, and supports files up to 50gb. You can password protect transfers and set the maximum amount of times a file can be downloaded.
Alternatively you can go a bit old school and encrypt the files on a pen drive and send by courier. I still do this as I have some family with broadband speeds from 20 years ago.
If privacy and security are your top priorities, which it sounds like they are, and you want a performance similar to OnePlus 8, go with the Google Pixel 8 with GrapheneOS. It’s more aligned with your need for strong app sandboxing and convenient Google services integration, etc.