How to invest in child’s future beyond college savings?

We have a 1 year old and want to set him up as ideally as possible. I do not want “his” money sitting in our savings any longer as it isn’t doing anything.

We already have a 529 College Advantage account open for him and will continue to contribute on a monthly basis for the next 17+ years so I don’t believe I want to dump a lump sum into that account.

Ideally, I would like to start investing on his behalf now to benefit him when he starts his own family (late 20s/early 30s etc). I feel like I’m stuck and have no idea what to do. I wanted to invest in some mutual funds but it sounds like a “custodial account” will transfer over at ~18-21 years of age which seems incredibly young.

Our personal finances to the best of my limited ability, my wife and I both get the max match from our 401Ks, we have a nest egg (several months’ worth of bills/mortgage etc), our debt is in our mortgage and my student loans, I have a Roth IRA that I am contributing to (a simple Vanguard Index Fund targeted date of 2055) but I am not MAXING our contributions as we aren’t that well off and money is tight lately.

I appreciate any guidance and suggestions you may have for this scenario. To recap: 1 year old, already has a 529 set up, looking to dump the lump sum into something that can actually accumulate a small amount of wealth not just sit in a savings account if possible.