The first part is just using JSONata with the custom function available in the HA nodes https://zachowj.github.io/node-red-contrib-home-assistant-websocket/guide/jsonata.html
Every event node is going to add a little overhead in memory and processing when an event arrives from HA. What’s that amount? Probably really small I’ve never looked into it.
Each call-service node would make an individual call to HA where a single one using a delimited list of ids would only make a single call. Once again probably not going to be noticed by the normal user, but definitely more overhead.
Here’s a link to how many service calls I could do without killing my HA instance. https://community.home-assistant.io/t/que-messages-and-send-with-specific-rate/334457/3?u=kermit