I'm somewhere in the middle. Whether it is grammar or spelling it doesn't matter, my point is to not to overdo it to the point that it is grating or become the focus (because the reader is more trying to figure out what is being said than reading). Basically, anything will be okay, but keep it subtle.
Also, just mentioning she has an accent is also okay. It's tricky though to keep reminding readers of her accent though without also becoming overbearing. But it's also an option in addition to using different grammar or spelling.
At the end of the day, this is what makes writing so hard sometimes - there's a million different ways to do something and choosing the best way isn't all that straightforward. For me, I'm always double-guessing whether I'm doing the right thing and wondering whether X instead of Y won't work better. Both works, but I guess this is where some books become award winners and receive widespread acclaim and others don't. I try not to stress too much about it as I just want my stories to be read. It's not so much about awards and acclaim. Others' mileage may vary.
All that said, what you've been doing so far works for me Rebecca. You already have a very distinct (historical) voice. I don't think it can be improved because it's already that good. But don't let me limit you. I have a lot of confidence in your ability to make anything work (even better despite what I think)!