For example there is no proper pdf reader that can sign a pdf and add or remove a page.
Unfortunately, pdf signing is problematic still on Linux, I use it as a daily driver and found a compromise with existing functionality. You can try okular, which is able to sign PDFs without altering them, but has a huge signature block and doesn’t permit adding a scan of a signature. My workaround: I created a stamp in the PDF reviewing tools with my signature, I can place that on the document and then sign it afterwards. Unfortunately, that doesn’t work for pre-signed PDFs as it will alter the signed version.
Alternatively, LibreOffice Draw can sign PDFs, but also can’t insert signature scans (yet, there’s an open feature request) and is sometimes not understanding when PDFs change to landscape, in general it’s not nice to render a many-pages document in LO Draw and hope that it won’t mess up the document upon signing.
For adding / removing pages, I agree - it’s a pity there’s no GUI application, but I have gotten used to qpdf / pdftk and they are quite powerful and more efficient 90% of the time. Still doesn’t excuse no GUI application, but it keeps me able to work.