So you want someone to write a plugin for Calibre to replace Zotero because you don't want to even slightly use it? Not likely to happen.
However, with no new software, here is how you easily do what you are requesting:
[1] Install Zotero 5.0. Customize it so its sqlite database is on a local drive (e.g. C).
[2] Wherever you create your Citation RIS files, do so. Example I used to test what I am telling you now:
[3] Import the RIS files into Zotero using "File > Import > RIS".
[4] When it finishes, since you do not want to use Zotero for anything, export the newly created Zotero items using "File > Export > CSV". Do not delete them yet; wait until everything has been imported into Calibre via ZMI to purge Zotero of items.
[5] Run Calibre.
[6] Create a "workbench" a.k.a. "intake" a.k.a. "in-process" Calibre Library, and switch to it.
[6] Install the "Zotero Metadata Importer" plugin. Restart Calibre.
[7] Set the Options for ZMI via its Options Tab. You want to be able to import anything, including .html files, and also items with no file attachments.
[8] Restart ZMI.
[9] Hover your mouse on the first Tab to read all of the ToolTips.
[9] Let ZMI automatically create its 26 Zotero metadata custom columns. Restart Calibre.
[10] In ZMI, use the top pushbutton to import your Zotero export .csv file. Finish processing them per the other pushbuttons per their ToolTips.
Now, you will have in Calibre all of your citation metadata from Zotero that came from your .ris exported Citations that you selected on the internet.
ZMI has thousands of users who also use Zotero for their thesis or dissertation or (in the case of university professors) their saved research documents, and who want the Zotero PDFs and metadata in Calibre as "ebooks".
Questions? Read the images attached to the ZMI Original Post, and all of the ToolTips. The answers to all of your questions are in the former.
Good luck.
DaltonST