Newer client apps will no longer support PMS versions older than 1.15.2.
As for the file size… If I remember correctly there was a major clean-up of the code, dependencies… reducing the build size when Plex introduced PMS version 1.15
Deleting the Plex Media Server app won’t delete your library (I believe that is except for the WD My Cloud environment which is having a very different setup).
If you want to backup Plex’ data first, this support article points out where to find that folder on your NAS https://support.plex.tv/articles/202915258-where-is-the-plex-media-server-data-directory-located/
That being said… while it’s a huge step from 1.14.1 to 1.19.5 this should actually work… might take a moment for Plex to upgrade the database all the way
