I have three kids, age 28, 23, and 21.
I'm no longer contributing to their 529s because my kids are mostly done with college now. One graduated and working, one a senior on full scholarship, and one taking an indefinite leave to start a business which is doing well.
We contributed sort of an arbitrary amount when starting. I eventually contributed as much as I could for a while.
I aimed to have four years of tuition, room, board, books, fees, and transportation for a four year public university per the College Board cost of college report. Once I got to that point in their early teens, I slowed down and just monitored things.
That amount was arbitrary but I figured I needed a target to keep myself sane. I FIREd while my kids were in high school and college. The deal I made with myself was that I would see how things turned out, and if I needed more money for their college I would supplement from my FIRE stash or go back to work. Things turned out fine and I have, in aggregate, somewhat overfunded their 529s.
Seeing how it worked for my kids, the 529s worked out very very well. Now with the ability to do 529->Roth rollovers (new law starting 2024), and with the scholarship and service academy exceptions, I think there are reasonable options for slightly overfunding 529s.
I personally don't think any other savings strategy works better *if* one has a good state tax benefit. Without the state tax benefit, maybe a regular taxable account is a competitive alternative. The only minor drawback to my 529 experience is that the expense ratios of the 529 are a bit higher than what I cold have done in a taxable account. That will vary by state, and in my case the state tax benefit more than offset the fees.
If the timing had been such that I would have still been working while my kids were in college, then I probably would have underfunded a bit and cash flowed some of it. There will always be the inherent problem of trying to drain it exactly to zero as the last kid gets their diploma. But again, the 529->Roth rollover option now makes that less of an issue.
I did also use the AOTC for each of my three kids. One can either pay $4K OOP each year for this credit, or take it from the 529 but make the withdrawal taxable. See IRS Pub 970 chapter on QTPs for details.
Note that there are some college expenses which are not 529 qualified; I generally paid these out of pocket. Things I remember in this category are the travel costs associated with college visits, college application fees, SAT fees, AP/CLEP fees, and transport to/from college if they go away to school. Not a large amount in the big scheme of things, but if you want to super plan then those would be things to note.
I was also able to buy several new computers and peripherals while they were in school because those are qualified expenses for 529 purposes if you meet the criteria.
I did make several nonqualified distributions when my kids were in college because I wanted to drain the 529s and was overfunded at that point and the taxes on their returns weren't too bad because they were low income. In retrospect that was probably a mistake. I think if I did it over again I would limit my 529 withdrawals to qualified expenses only. Those would include 529->Roth rollovers, rolling it down to younger siblings or cousins, using them for graduate school, or keeping them as an educational trust fund for the grandkids if those ever appear.