If you want to try nextcloud use the nextcloudpi project. It takes care of everything you need to set it up.
I use seafile. The docs are so so, the support is not great but the product did not failed once for two years (with my server being on linux (docker) , clients on linux, windows and android)
If you are looking for a commercial (not opensource) self-hosted dropbox alternative, FileCloud (https://www.getfilecloud.com) is great. It comes with great support and has clients for windows, mac and linux.
Full Disclosure: I work here.
I have been a very happy user of Syncthing over Wireguard (connecting all my devices back home). Adding to that a VM of "OpenMediaVault" for NAS/Backup, PiHole to catch what Ublock misses, and music streaming via FunkWhale, I have managed to host a good majority of my own data.
Is there anything that has a client for Linux, Mac, and iOS? Not just a website, but a dedicated app.
A Synology NAS + Synology Drive has replaced Dropbox for me. Drive isn’t quite as polished, but I have effectively unlimited space, and it’s more private.
What about not fully Dropbox alternatives, but simple solutions to have a backup of files online and sync those files with different computers?
For those of us that have the same aversion to setting up an entire web server for a simple browser-based access to their files as I have, I recommend also having a look at https://filebrowser.xyz
I have an instance running at home (reachable only via VPN) that simultaneously syncs the files via Syncthing, and the combination of the two is surprisingly effective.
Adding to this this project currently in beta, a simple self-hosted private Dropbox with E2EE: https://www.duple.io/en/
Syncthing is fantastic, and is mostly private. With a little effort, you can make it completely private by setting up your own discovery and relay server (though the docs on how to do that could really be improved).