I have created an Excel tool that automatically generates a Sitemap JSON from a list of up to 500 urls (can be extended by changing the formula as long as it complies with the maximum number of 32,767 characters per cell in the generated cell).
To work you need to have at least Excel 2016 (when the TEXTJOIN formula was introduced) and download the file by clicking on File/Download. It will not work in Google Sheets directly probably because it does not support the Excel TEXTJOIN function or the TEXTJOIN function within the CONCAT function. If anyone is able to make it work in Google Sheets please let me know. Hope it helps.
Webscraper Sitemap JSON Generator from list of urls