Do not sacrifice simplicity on the altar of scalability
The ideal solution will be "almost trivial" to implement once you know how
Flat file CMS - no database
Write in markdown (i.e. plain text)
Easy upload / authoring from desktop or phone
Upload from insecure device (maybe with approval/authorization from secure device?)
Content should be as transferable as possible - minimum proprietary syntax
No need to implement all admin functionality in CMS UI - admin is assumed to have read/write access to the web storage anyway, so that can be used for file deletion, editing templates, etc.
Key functionality is fast and convenient authoring/editing of content from a wide range of devices.
Intended for single author or small team where all authors are in touch with admin
No JavaScript for CMS user or site visitors - HTML and CSS only
Minimal data collection from site visitors - no cookies, no tracking, no popups, no permissions
Security implemented via Apache password protected folder(s) and TLS (i.e. https) - allows login via URL shortcut, e.g. "https://johndoe:MyPa55word@tedz.eu/cms/"
https for everyone - CMS users and site visitors
Philosophy / Inspirations
"folk programming"
"user developer" - Terry Davis
"CMS from scratch" a la Linux From Scratch
Type-in games from the 80s
No tool lock-in - write your content in a format you can take somewhere else whenever you want