LibreOffice has a command line option to convert files involving the soffice(exe) file and -convert-to and related parameters. I suggest making a copy of a dozen or so of your files into a folder and experimenting before trying the larger batch.
Here is another answer with some information about using the command line and which parameters to try.
Sorry, I’ve never done this myself, otherwise I’d give more specific info. Maybe there’s an expert on here with a better answer.
I’d emphasize being smart with how you go about the conversion, experimenting first. Work from a copy of your folder of files in case something goes oddly wrong or you choose the wrong command line parameter by accident.