My Idea is to make supplier payments from ERP by calling the BItcoincore testnet first and then later on the mainnet. Any idea/documentation/thread/input is highly appreciated.
Are you aware of this Bitcoin Core limitation?
I would not recommend doing that. Bitcoin Core is not well equipped to handle both large wallets, and a large number of wallets. Either it will perform very poorly or it will consume more memory than you have.
It is not possible to get a wallet's balance without loading it. Loading a wallet can take a lot of time, and constantly loading and unloading wallets will waste a lot of time. When a wallet is loaded, it is entirely loaded into memory and the entire wallet is iterated at least once.
Maybe you already know, but I want to be sure:
You will probably have to run bitcoind (i.e. the daemon, not the wallet UI).
When you start wallet UI (Bitcoin Qt), the daemon (
bitcoind) also running on background. But OP should use
bitcoind directly for server.