the problem with these local approaches is that Safari would destroy all the data after one week of no usage. So we need some way to backup the data either on file locally, or on a server, which is not easy and not part of these approaches.

Disgraceful that Apple does this in the name of privacy, when this implies less privacy and pushes people to either build apps (with much of Apple's economic gain) or some centralised database (at the expense of data protection).

Is there any browser based databases with backend synchronisation and E2E encryption included?