i have multiple google drives synced right into my file manager...like i just click it, it mounts it, and drag stuff in and out as if it were local...i'm on debian with gnome. dropbox works the same way. obviously icloud and onedrive may be more difficult, but i'm pretty sure there is something formsyncing up onedrive, but i choose to disable one drive on all my windows devices.

Seafile works well on linux

Tresorit has a Linux client.

Has anyone tried cryptpad.fr. I'm considering it, but I have yet to try it.

I use kDrive and it works well with Fedora. Infomaniak, the company behind kDrive, is from my country, Switzerland. It uses a lot of renewable energy and the heat from their servers is used to heat buildings in my city.

I use my own NAS along with syncthing to backup and sync stuff across my phone, laptop, and desktop. Before that I was using mega.nz with its native Linux client, which worked fine sans a weird issue where it'd repeatedly transfer the same file forever.

Way back I also saw a paid 3rd party Linux-native app that supposedly works with all the major personal cloud carriers, though I never ended up using it and have long since forgotten what it's even called.

with rclone you can mount cloud storage as a folder

I am currently using InSync on 64-bit devices and Overgrive on 32-bit devices. Overgrive works just fine on 64-bit devices tol but Insync is slightly more userfriendly.

I use Google cloud with nautilus, and before that I used google-drive-ocamlfuse on my Chromebook with custom firmware. All this just so I don't have to use their stupid website.

Last time I tried, Dropbox has the best linux app. It even supports LAN syncing and integration with nautilus (ubuntu default file manager). However I need to move to onedrive because dropbox is just too expensive

Try nextcloud