First and foremost, take care of your own retirement and investing needs first. The best thing you can do for your kids is to remain financially independent until death, and not put them in a position where they feel they have to sacrifice what their own young family needs to support you.

Once that’s done, I’m a proponent of over saving. Too much is better than too little, and the worst case for unqualified 529 withdraws are taxes (which you’d be paying anyway if in a taxable account) and 10% on earnings only, not principal.

So if you ended up with $150K too much, assuming half was earnings and half was principal, the penalty would be $7500. Not ideal obviously, but that’s still $142,500 back in your pocket, minus the same taxes you’d be paying anyway of course.

As far as worst cases go, that ain’t too bad.