You can place your address & private key(s) in cold storage and then integrate a service such as
https://getblock.io/ (they have a free tier) in your backend to track payments to this address in real-time.
Or even better, make a master private key and master public key, put the master public key in your backend code and programmatically derive an address for each customer using BIP44 (I know you don't want to use NBitcoin, but it's good for just that purpose), then use an API to track payments to each of those addresses. You will be able to derive the corresponding private keys offline using a software wallet such as Electrum.