The CW used to be to amend planting holes but now many sources advise against that, as entities installed this way are examined post-mortem, vs. others planted in unamended holes. Creating a zone of different soil around the roots of something in the ground can cause issues since moisture will move through at different rates, either more slowly, creating a soggy bowl, or more quickly, creating hydrophobically dry particles/zone around the roots. Either is bad, and roots often don't make the leap into the native soil, dying an early death, or failing to establish at all. At the bottom of the hole, I like to use something to spike some holes straight down, like a big screwdriver or dandelion fork. This should give roots (and water) some easy places to go straight down. When I look at stuff I've planted on the past on google earth, it's there, doing great!
I've not had that type of palm before, but for a root ball like that, usual procedure that I follow is to use a shovel or saw to remove the bottom portion, for repotting or putting in the ground. How much would depend on what it looks like out of the pot. Usually a few inches is enough to remove the 'circular strangle' that roots usually get into at the bottom in a pot. Then roots are free/able to roam as far as they naturally would much more easily. Often, once the 'pancake' of solid, strangled roots is removed at the bottom, the inside is relatively hollow. If so, I would remove as much nursery soil as possible and re-pack with native soil, then install in hole. Using something for temporary shade/windblock can help if heat/sun or wind are a concern at first.
