bidirectional synchronization between IOS 14.7 and NAS (Synology)

hi everyone, sorry for not speaking better english. i just switched from android to ios. the windows - mac jump happened two years ago. between the mac and the NAS synchronization works perfectly.

my subject: I mainly want to bidirectionally synchronize my recent images between the iphone 12 mini and the "new photos" folder of the Synology NAS.

i-cloud & the like, ds photo, ds drive are not options, because either paid / insecure, or not bidirectional and / or not automatic .. the Apple store apps that I found are supposedly not automatic .

in a second step, i want to synchronize all my old images between the NAS and the iphone. preferably so that they can be displayed in "Fotos" on both apple devices.

my goal is to be able to view, sort, move (different folders), modify, delete my photos on the device used at the time without having to worry about synchronization.

thank you for your help, in case you answer me in french or german i will understand you more easily