And there is a solution...
Updated: Jul 30, 2019
... and it's IndexedDB.
What is IndexedDB?
IndexedDB is a no-sql (or document database) that is a part of every browser. Unlike localstorage or session storage (which peak at about 10Mb), IndexedDB stores data up to 50% of available disk space - That's 100Gb on a Huawei P30!!!
That kind of storage is what we need.
Anyway, no idea how it scales, even googling gives no answers, we're going to build a test application and hammer it to see how it works...
Happy New Year!