Maybe, very maybe, this will be possible in a dirty way with a Siri Shortcut if bunq offers to get a total of the transactions today or a list of all transactions today that can then be totaled in Shortcuts. When it gets this info, make it write to a local agenda. Then add the agenda complication, which is also the only complication that actually updates in realtime, to show that information to you on the Watch. Then an automation to have it ran x times a day, if Apple has fixed the damned notification before run anyway.
Alternatively, if you have a Mac or Linux (can be on a server too) you can definitely do this with the API + cron job and have it write to an iCloud-agenda (or whatever you use) that’s shared with your phone. That will work 100% with a bit of programming. Last but not least, should you have Pythonista then you can run the whole bunq Python SDK on your iPhone lol - and it integrates with Shortcuts. So that’s another route.
Very dirty solution, and it may mess with displaying other agenda items (buying a secondary agenda app for your regular items would be an idea, such as Fantastical), but figured I’d throw the idea out here if anyone really wants to have such a thing for as long as bunq doesn’t offer it, heh.