To help with frost, use a layer of 2" foam, 4'x4' with a hole the size of the post in the middle, place this just below grade, insulating the earth around the post.
I find it odd that we dig a hole and fill it with crushed stone, it is like a bucket to collect water. But if the soil is well drained the bucket will leak out the water before it freezes, maybe. This would not be the case in loamy or clay based soils, here a drainage ditch to light would be needed or use the same soil to back fill.
A trellis, is this in a garden? If it is anything like my gardens things change and in 20 years I suspect it will be removed and replace with another something. I would expect a life of 20 years. The bigger issue, beyond the life of the post is the frost heaving, address that first. Even if you cast a concrete pier frost could fowl it up. The post could act as a cold sink and aid in freezing the ground around the post, so keep it as low to grade as possible.
