Your follow-up about the admin handling calls seems like the key question: which decisions can they make without needing your judgment every time?
I’d try a one-page guide with three outcomes: book the video call, ask for one missing detail, or check with you. Keep the firm boundaries specific—service area, work you actually offer, and dates you can realistically accommodate. An unknown budget could go into the conversation notes rather than automatically ruling someone out.
To test the guide, you and your admin could independently sort ten past inquiries and compare where you disagree. Those differences show where another rule or example might help. What currently sends the most calls back to you: unclear scope, budget, or deciding whether the job is a fit?