If you're willing to run a Node.js app yourself, I am working on some updates to Jarbunq (https://github.com/igoramadas/jarbunq) to add a new action "balanceTransfer" that makes it possible to transfer a % of the balance from one account to another based on received callbacks. I'll have that feature pushed to the repo tomorrow, as I still need to do some more testing.
Apart from that, it is already possible to make payments, send emails and push notifications based on payments and events happening on your account, pretty much like IFTTT but configurable via a JSON file. Draft docs are here: https://github.com/igoramadas/jarbunq/wiki/Eventhooks
This might be a not very popular opinion, but I would actually prefer if the bunq app kept just the essentials. Leave such automation / niche features for external devs to implement :-) That's the whole point of having a public API, right? OK maybe they could have an additional app like "bunq Labs" with such features, pretty much like they had "Slice" for group payments splitting, or Instagram has Layout and Boomerang, Facebook has Messenger... But please, keep the official bunq app as lean as possible :-)