Trying to snake a clogged external tower style drain/standing waste overflow and can't seem to go further down
I know you don't want to hear this, but as a plumber/maintenance person, I will tell you this - There is no science to your dilemna. When we are called to your issue, we will try the same things you did, only we never use chemicals before snaking. I only use chemicals for under-concrete-pad-or-crawlspace hopeless clogs, because I use acids only available to licensed plumbers, that will melt your pipes if left to set too long.

Other than that, it's persistance. Keep jamming coat hangers, augers, electrician fish tape, whatever you can dig out of your truck or closet, into that booger until you pull out a plum. Without getting underneath the plumbing and physically seeing your piping, there's no way to know what you could be hitting.

There is a water jet drain opener, but that could get messy fast if you've never used one. Google "water jet drain cleaning", maybe Youtube it as well. The tool is about $10 at Walmart, and only needs a garden hose.