Kevin 🇩🇪🇬🇧 You can only access this info through the API, so the simple solution would be to generate an API key and use that to retrieve the amount. The API key can be set so that it only responds to queries from a certain IP address. If your server has a fixed IP address, that could give you a bit more security if for some reason the API key would get stolen (however, if your server gets hacked then the attacker could just use that to send queries).
Alternatively you can use OAuth, but that makes the authentication flow a bit more involved. Your server would then be the consumer of your OAuth application. Good thing about that is that then your server has only limited access to your bunq account. Enough to retrieve the account balance, but not enough to send a payment on its own.
Having implemented both ways myself with the PHP SDK, I would however call both of them not very simple. It still requires a good understanding of the authentication process with the bunq API, which I found unintuitive at the beginning. However, once you get behind how it works and you have set up the authentication successfully, the rest is pretty quick and easy and you can do all kinds of cool things.