The way I would look at it is how far you are progessing toward multiple goals. One goal is to pay off the mortgage, another is to have investments worth X, etc. If you are "undoing" your progress on one goal (i.e. pulling equity out of your house), it's going to be more painful because you've invested yourself in getting to this point.
You asking this question has been a little helpful for me, too, because I am in a similar boat. But rather than equity, I'm thinking about cash flow. I've got a 15 year mortgage right now, because when I bought my house a year ago I was thinking that I would want to be mortgage free at 50. But as I watch my principal go down by a substantial chunk each month, I'm wondering if I should have gotten a 30 year mortgage and invested the difference.
The "shockingly simple math" says to add principal repayment as part of your savings rate, right? But that is half of what I'm saving right now and it's only earning me 3.5% versus the other half that is going into 401k... Anyway, I still haven't figured out how I feel about the different options. Whether I'm happier with the security of the mortgage being paid off sooner or whether I'm not going to have enough 'stashed because I missed out on market compounding. That said, once I know where I want to be in the future, I can work back to determine what I should be doing now. And I think that's what you want to do, too. You shouldn't "undo" too much of your mortgage paydown that you don't feel secure enough as you approach ER.
Maybe this was too much of a ramble to be any help, but I thank you because it's helping me work through similar issues.