I think it would be better only first post at 100% and all others (including old pages) at 50%
You can try achieving this by removing the existing used CSS code and using the custom CSS code solution posted in the any of the following topic.
https://colorlibsupport.com/t/how-to-make-only-one-post-full-width-and-the-rest-in-grid/#post-58491
https://colorlibsupport.com/t/a-few-questions-how-to-make-only-the-first-post-full-and-regarding-font/#post-59417