Since ODS generation can cause issues (https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=23685), and Excel generation is not currently possible, why not using libraries such as https://sheetjs.com/ (and other tools like https://www.papaparse.com/ or web workers for big files?) to convert the generated CSV to Excel or ODS in the browser ? It could be used for reports but also other CSV generated by Koha. We will try this approach in my library, as a local customization.