Tree looks like it's planted too deep--no sign of the root flare. And I think the staking is overkill for that small tree.

find and expose the root flare

https://duckduckgo.com/?t=ffab&q=root+flare&iax=images&ia=images

and wild guess here.. stop fertilizing it.. it appears to favor growing leaves.. faster than it can thicken the trunk.. and the only reason for that.. would be excessive fert ... either before you got it.. or since you planted it..

trees rarely if ever need fert .. do a soil test.. if you suspect something is missing from your soil... but otherwise.. they are not children that need to be fed..

ken