If you are using Kubuntu you will have Dolphin installed. It is very good at handling remote file systems and I find I have no need for separate ftp/sftp/ftps software. If you do not already, now would be a good time to switch to using ssh keys. I use ssh-agent so I can passphrase protect a key and still unlock it only once per session. A bit of set up but a time saver in the long run.
KDE, so where KDE software is good try it.: Digikam, Krita, Kate, Falkon, Konqueror, Akregator, Kmail, KDE Connect (if you use Android devices), Okular (should be the default) are all excellent apps (not all what I use, but that is down to preferring something else) and there are more: [
apps.kde.org...]
I have not found Calligra (the KDE office suite) to be much good, but installing it does let you preview office documents in Konqueror (tabbed, paned, file manager and browser) and you may need it isntalled to see office document thumbnails in Dolphin.
You might have stability issues with nvidia cards. I found changing the compositor rendering backend to XRender fixed it. Probably worse performance, but not noticeable with desktop effects. On that subject, KDEs effects, especially the desktop cube, are great fun.