Years ago with the dogecoin cli I built a service in python that would act as an arbitrary between two parties and allow users to send money much like paypal.
Have you ever used the bitcoin-cli tool to manually build a transaction?
I wrote a guide on how to do this for transactions involving inscriptions, but the same concepts apply to non-inscriptions as well https://github.com/vicariousdrama/howto/raw/main/Using%20Bitcoin-CLI%20with%20Ordinal%20Inscriptions.pdf