We don't know where you are gardening - zone 6 traverses the North American continent.

We don't know anything about the site, short of the plants that did not survive.

Give this audience as much specific information from which to begin offering advice. Where do you garden (city, state, province)? Soil pH, rainfall, average temps in summer and winter, plant communities endemic to your area, etc. etc. etc.

What is the history of this property? Do you live in a newly constructed area, where original soils were not respected and you are trying to garden on C horizon soils? Or something else?

FINALLY: show us what you are dealing with. Not just a picture of a hole in the ground where previously plantings failed, but the entire context that you are struggling with.

All these things give participants here a better picture of the situation, and a far better opportunity to help solve your dilemma.

build a berm with good soil ... proportional to plant size .. and plant high ... nvm.. im just going to go find what i just typed in that other current post.. and paste it here...

and i would start with a rather small cheap tree.. not some 12 foot transplant ...

and perk your spot if you havent already ...

ken

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clay is usually very fertile.. and trees have no problem with such.. when they grow into it naturally ...

our problem is transplanting into the bad situation .. and getting it to live long enough to put its roots where it wants them.. and where they can cope ... the usual suspect in failure.. is watering a hole that doesnt drain properly ....

it is usually suggested ... that you plant high in good soil... lets call it a berm .... so the plant can be watered yet excess can drain away ... giving the plant that year or 3 it needs to grow the root mass to survive ..

good work on perking ... thats my usually starting point ... at least you know what you are dealing with below the surface ...

second.. never amend a planting hole .. it does nothing if its basically a ceramic pot that doesnt drain ...

lastly... trees never really stop growing at some magical height or width ... so when you say... something will fit there .. it is really only a question of how long it will take.. to outgrow it ...

perhaps a pic of the situation .. will get you some suggestions besides those you mention ...