Your math is all fine.
But: A recent statement might just be backwards looking - be sure to estimate dividend and capital gain distributions for Q4'19
There isn't any special reason to stay below $40k income ("just to make sure I don't trigger any taxes"), even though that is what you estimated for ACA purposes. For the following, I'm assuming you are a household of size 2 for ACA purposes:
If you did no Roth conversion this year, and income came in at ~$34k (206% FPL for household size = 2) you would get a refund of premiums already paid of ~$950.
In other words, doing a $6k Roth conversion to get income to exactly $40k (243% FPL) costs you $950 over doing nothing, or a tax rate of 15% or so.
On the other extreme, let's say you did a Roth conversion of ~$27k, pushing income to $61k (370% FPL.)
This would generate $500 in income tax, which could be offset with the Foreign Tax Credit for the $500 foreign tax paid - net result is $0 income tax.
However, it would also mean you needed to pay back $2,600 of ACA premiums (the limit for repayback, even though technically you would owe up to $2772.)
https://www.gocurrycracker.com/obamacare-advanced-premium-tax-credit-repayment-limitation/That $2600 on $21k of income (over the 40k mark) is a 12.4% tax rate.
Summary:
do nothing and get a refund
convert a little bit and pay 15% tax
convert a lot, recover the foreign taxes paid, and pay back aca premium credits at a 12.x% rate