It sounds like you've done your research. Certainly you know that the interest rate used must be no greater than 120% of the mid-term AFR in either of the two months prior to initiation of the SEPP. 120% of the mid-term AFR for this month is 2.03% per
https://www.irs.gov/pub/irs-drop/rr-19-26.pdf, so you can fiddle with the interest rate if you want.
Using the calculator you mentioned, an interest rate of 1.3%, age 54, and the uniform life table, it looks like the amortization method throws off about 3% and the annuitization method throws off about 4%. Which table were you using for life expectancy, and which method were you planning on using?
I haven't done an SEPP personally because it doesn't fit my situation currently. But I considered using it before finding out about the Roth conversion ladder so I'm fairly familiar with it in principle.
I am not certain, but I think if you started the SEPP at age 54 in January 2020, you could make annual withdrawals in January 2020, 2021, 2022, 2023, 2024, and January 2025. Then when you hit 59.5 in summer 2025 do whatever you want. So it would be six years of SEPP withdrawals, not seven.
People worry about the calculation but I think it is pretty straightforward. Try several online calculators and make sure you're confident in the result. It would be nice if the calculators all agreed.