I put all my content in Dropbox and use
DropSync to sync a specific folder onto my Onyx.
Alternatively I use Telegram to transfer files too. I created a group of one (create a group of two and then kick one people out) and put all the files/message inside that group, using Desktop or my phone. Then on Onyx I installed
Telegram X for receiving message/files.
For notes I am trying
Notion. It is a bit complicated but very powerful.
See if that works for you.