@R2D2
"I go to the supermarket, pay for my groceries. I've selected that for this merchant I'd like to use account Y ; so I enter my PIN, the system checks if there are enough funds on Y and executes the transaction."
The rule (and underlying assumption) is not the same for everyone because everyone has different budgets and not everyone ties individual merchants to budgets: I can pay for something at a gas station which sometimes falls in the budget 'car maintenance' and sometimes it can fall in the budget 'daily expenses'. I can pay for something at a department store and depending on how I do my budgeting, it could fit any budget. That means it's no solution for everyone and you still need to do corrections. It mostly seems to be a solution for thematic shopping (groceries), but for that there are other and better solutions.
"I think 90 % of my transactions are always from the same merchants, so these allocations to the right budget/IBAN account would be easily automated. Right now I do this 'afterwards' in a budgetting tool."
My first tip would be to do some soul searching about your budgeting approach. You don't want to select the right budget upfront and you also don't want to budget afterwards or correct afterwards by transacting from the 'correct' budget to the 'abused' budget. You don't want to use the secondary account features and remember a secondary pin, nor do you want to use the two mastercards for this purpose (in that case you would have three cards on which you could use the same pin and you would be able to transact using your three most common budgets). Lastly you also don't want to order extra maestro cards and pay for the extra convenience (I did this btw because I also got tired of switching between my two most-used budgets).
Where does bunq's responsibility to tailor their app to your needs end and where does your own responsibility start to use the possibilities they already offer you?
My second tip would be to think about the usefulness of using a budgeting tool (as the budgeting tool is often mentioned as a reason why the transactions all have to be correct and there can't be budget corrections between customer accounts). I've tried it years ago and it did not work for me. Where your budgeting approach fails is that you already cannot be bothered to select the right budget upfront. If there is no money because your budget is empty (or you see it go down fast), then you will control your spending habits. Budgeting happens in the store and not afterwards enjoying that pack of beers you should not have bought while you're updating the budgets. Secondly, budgeting tools are not very actionable anyway because they don't tell you what the individual items were you bought. You just know what you spent in a certain category. If you want to do actional budget analysis you need to keep your receipts and manually check which groceries were essential and which ones weren't anyway.
I believe the problem you're asking bunq to solve is not something they can or should solve.