Did you calculate the monthly payments on the heloc and can you afford those, on top of tax bites? 20k seems to be a little more than 200/month repay. It only goes up as you take more. So you’d want to play out your calculations for each of the 6 years. Including taxes and any effect on future FA.
Paying interest alone seems to keep monthly down at first. But you’re left with the big nut, which remains years later. Then you dig into the 401k, at what could be a vulnerable time. You might continue working or something else could happen. Without one income, could you or your spouse manage? You could have that 100-200k debt, lower income, and a higher need for those 401k funds.
Sorry. That’s the problem with large $ forward financing ideas. You know what might work today, not what could happen and projecting the strain. You’re closer to retirement than a lot of parents on CC.