What to plant a fruit tree in ?

I suggest that when you un-pot them, you remove as much of the compost from around their roots as possible and spread their roots around the planting hole.
Absolutely don't let the roots dry out while it's out of the pot - you must re-plant the tree within a couple of hours at most.
"Bare-rooting" may seem harsh - and will damage some of their roots - but it's better in the long run. **
Roots hate to grow from rich compost into poor soil; many container-grown plants that are simply taken fro their pot (rootbacll still intact) don't like to grow outwards out of their compost because of poor or dry soil being less tempting to the roots.
Container-grown roots also tend to grow in circles instead of outwards, unless you spread the roots around the planting hole.
I'd fill the planting hole with an equal mix of your native soil and compost - then water immediately after planting to force out any air pockets (roots don't like to grow near air pockets) and to settle the soil. Hopefully the planting hole will sink slightly, which will act as a basin to stop water running away during the drier summer months.

Also add an inch thickness of compost as a surface mulch, to help retain moisture and to provide a gradual release of nutrients during the growing season.

**

Slightly off-topic;
I recently planted a container-grown apple tree - on MM106 rootstock; the following is not advisable for the weak rootstocks (nor advisable during the growing season), but is provided for curiosity's sake!

When I took it out of the pot, the roots were severely tangled.

What did I do?

I got out a saw and cut off the bottom 2/3 of the root system.
Then planted it a about three inches deeper than the old soil line (but with the graft still just above ground level).
I did not prune the top of the tree; it'd be too much stress.

My expectation is that:

Firstly, the tree will be really angry with me and - like a pruned tree - will send out lots of new roots to replace what was cut off. These new roots will grow straight and deep, rather than circle around the planting hole.

Secondly, the tree now being planted a few inches deeper will encourage the rootstock to send out new "feeder" roots just half an inch below the soil surface. These will not grow in a tangled fashion, since they will not be constrained by a pot as they try to grow outwards during this summer.

Thanks FB, I will set to this weekend before the growing season starts.