an ESPP is a taxable account and you would apply it there (at the end), if anywhere

And that really depends on the terms of the ESPP. If it's a big enough discount, you could prioritize it over some other parts earlier in the flowchart.

The flowchart shouldn't try to account for every scenario, it should be a starting point for most people. I honestly think it's too complicated and prefer things like the Money Guy's Financial Order of Operations (FOO), because that focuses on principles instead of following a flowchart. Here it is in text, with some personal notes (image below):

1. Deductibles covered - highest deductible, so you're not screwed if something pops up
2. Employer Match - free money; ESPP is also "free money," but it locks up your money, so I'd push it off
3. High Interest Debt - they have a metric for this, but generally anything other than housing >6%
4. Emergency Fund - 3-6 months expenses, more if your job is sketchy, less if you're dual income and can survive on one
5. Roth & HSA - specifically talking about individual, tax-advantaged investment plans, in case you're not in the US
6. Max out retirement - bit of a misnomer, they say max 25% of income
7. Hyper-accumulation - 25% of income
8. Prepaid Future Expenses - fund kids' college funds, prepay cremation/burial expenses, etc
9. Low-interest debt - mostly just mortgage for most people

Even here with a simplified priority system, the ESPP could go in 2 (if it has a low max), 5 or 6 (if the discount is good enough), or 7, or even none of the above if the holding time is longer that you feel comfortable with. It really depends on the company, terms, and your personal risk tolerance. You can go above 25% of income toward retirement, but they actively discourage it because that could put you into miser territory, and recommend giving if you don't have anything useful to spend excess on.