Scrypted is what you want. I’m using it to stream an Amcrest AD410 doorbell camera to my NAS and forward it to HomeKit Secure Video with a local copy of the video saved on my NAS. It’s rock solid. It uses a plugin architecture and HKSV is just a Scrypted plugin (as is saving the video locally). The developer is very responsive.

https://github.com/koush/scrypted