It's hard to say without a pic, but once a tree is declining like that, it's virtually a goner and nothing will save it.

Your "arborist" is full of bovine excrement, there is no $200 magic liquid that would revive it at this stage. I also can't envision how its own branches are choking it, unless something is actually wrapped around the trunk somehow.

I'm sorry to say, you need to just take it out and plant a replacement.

The only thing that 'magic' liquid will do is make his wallet fatter. Another snake oil salesman.
Branches interfering with the flow of water?!! What's that all about?
Time to take the tree out and not see that guy again, even to remove the tree. Sheesh!
Mike

maybe i'll get a second opinion. so i guess im not sure what i could have done differently. how does a large established tree just vanish in two years. kinda frustrating. makes me worry some of my other trees may die if it is a disease. any ideas how i might go about diagnosing the problem?

how about a pic

ken

I'm betting buried root zone, excavation/construction, or compaction, in that order. Probable a combo of all three. Wild card causes trunk damage, fire damage, drought after some other insult, or chemical. But I'm also betting we never find out as OP doesn't seem too keen on answering questions.

Arktrees

No "maybe" about that second opinion. I highly doubt your "arborist" is actually certified, and if he is, he should have his certification revoked. You did check his certification, yes?

Here are some pics

Here's another. Let me know if need more or what to take a picture of.

Its basically already dead. Cut it down and move on.

And don't ever consult that arborist again.