You do not have to invest in a state plan. You can set up a 529 through an investment firm like Fidelity, T Rowe Price, Wells Fargo Investors, etc.
You can contribute to the parents’ 529 and still get the tax benefits. However, you are losing control over the money. If you set up your own 529 for the grandkids, you retain control. So, if you are worried at all that the parents may take the money out of the 529 or not otherwise use it on the grandchild’s college expenses, then it’s best to set up your own 529.
Grandparent 529 accounts will be “better” than parent 529 accounts in the FAFSA formula beginning in 2023-24. The money a grandparent (or anyone other than a parent) pays toward a student’s schooling won’t be counted. I wonder how long that will last - it doesn’t really make sense in terms of what the government is trying to do as it transforms financial aid. I don’t have a crystal ball, but I wouldn’t be shocked if that rule changes before our baby grands are in college. I also don’t know what Profile schools will do when this FAFSA rule kicks in, but I would not be surprised if they count the non-parent payments for institutional aid. And finally … if your kids have high salaries, they may very well not qualify for aid. In that case, it doesn’t matter whose 529 account the money is in.
Since you are using the “plural” - one consideration could be future grandchildren from the other “kids”. If you maintain the 529 under your name, it will make it easy to re-designate your grandparent 529 to other kids’ children, in case the first grandchild gets great scholarships and doesn’t draw it down, or “life happens” in other ways.
My FA advised I contribute to the one the parents set up for the grands; one consideration was one less thing to divvy up when my stuff needs divvying.
So far, since I’m “just an aunt” or great aunt, H&I just write checks to the parents & kid. We trust the parent to use it as needed and haven’t gone into the weeds with the 529 or coverdell. We will explore that when we are grandparents, perhaps.
One option is to use funds for private/parochial school fees (elementary through high school), another is that parents might decide to change (or double-down on) their career and pursue a second degree, masters, etc.