I’m sure you have a reliable, and honest mechanic. But I am a skeptic. If they tell you the head needs milling, ask them to show you.
Keep in mind, I’ve never asked myself. So I’m not sure if that is something they would do. the last time I had a head gasket done, I was in my 20s and not a quarter knowledgeable on auto repair as I am today. My 1986 volvo with 250K miles on it cost me $2500 for the head gasket repair down in Augusta, GA in 1993.
If I had a head gasket repair today and they told me the head needed to be milled, I would bring a metal straight edge and ask them to show me the warping.
Here’s a good youtube video to show you what they are looking for.
Over time, you’ll learn to stop and search out a repair on youtube. I find I’m able to do at least 50 percent of repairs to my car just by searching. Some fixes are so easy you wonder why they charge hundreds of dollars! But there are a lot of repairs I don’t care to do if they are too messy, complicated, time consuming, or require me to buy special tools.