I know not all people use features like this, but I am hosting a Nextcloud application on my computer and have downloaded the nextcloud application for Android (also available for IOS). I have simply setup sync to the DCIM folder (including screenshots), WhatsApp Images folder and WhatsApp Video folder. As soon as I receive any picture/video, same gets uploaded to my Nextcloud application. This can be accessed through my mobile application for nextcloud or from webportal directly. As far as WhatsApp chats are concerned, they get backed up on GDrive. Chats anyway won't take too much space in the GDrive.
PS: There are many other options available like Nextcloud. One just needs to search. As far as I have read about Synology (not used it ever), it also has a backup feature for media files.
