If anyone is considering implementing something like this, PLEASE consider making it a complementary web-version to Money Manager EX: https://www.moneymanagerex.org/
MMEX is fully open-source, cross-platform, has a great android client and already syncing with dropbox etc.
But it really lacks a web-interface and nicer sync.
A while ago they experimented with something, but it was abandoned again:
So that could be a start as reference.