OP here. Thanks again, everyone. Some thoughts ...
With our oldest we would definitely be looking at both FAFSA and CSS/Profile schools since somewhere like MIT is plausible, while State Unis and others need to be on the list. We have to figure what strategy to pursue when either method could be used. In the coming years we'll have choices about how to contribute to Trad/Roth/taxable (and later to withdraw or transfer between these), so there is flexibility, but there is a playoff between income and assets, where decreasing one increases the other. That's tricky to plan for, especially since the CSS/Profile method is somewhat unknown. Taking loans (including HELOC) is a way of spending your own money, plus interest, without it counting as income or assets, but that involves extra cost, risk and complexity.
There are college planning discussions all over the internet, but one additional ingredient for early retirees is that you basically have a fixed budget. If your kid gets into an expensive school, you may simply not be able to afford it (same for most people) whereas some still-working people may decide they think it's worth it, even if it means working longer.
I feel like we need to fix a budget, e.g. $20k per year (not sure the number), and tell the kids to work with that, or take responsibility for any excess, though I'd rather they'd not have debt. Places like Harvard/MIT are much cheaper than that (according to Net Price Calculators using our anticipated numbers) and local State U is cheap (especially with kid staying home) and some other types of Unis are cheap enough, but most unis seem unaffordable, so we'd cut them from the outset. You have to be discerning consumers and not pay too much.
We plan to try to use the Simplified Needs Test (though it's only for FAFSA) since AGI<$50k is easy to plan for when withdrawing from retirement accounts, and having modest expenses. (The even lower AGI for zero EFC is plausible but a stretch)
But the big difficulty is qualifying to use form 1040A by having no items in this list
FinAid | FinAid for Educators and FAAs | Simplified Needs Test Chart
So you can't need Schedule D, meaning you cannot buy/sell(/own?) any asset which could generate cap gain/loss, so only bank accounts or money market funds in taxable, which may not be otherwise optimal.
No reportable HSA activity. A dormant account is okay, but no contributions or withdrawals.
No Line 21 (Misc income) so if you get ten bucks for jury duty, you could lose thousands in college aid. Etc.
(Alternatively qualify for Free/Reduced School Lunch.)
It's worth planning for this, but things can go wrong.
From the comments, it appears Roth conversions are out, and that generally you don't want to plan on needing any special consideration, even though you may ask. You may be better relying on what a formula/algorithm outputs, rather than having a human scrutinize your situation.
For a while we saved in 529 plans and now have $50k per kid (3 kids) but at some point I realized that from a net worth perspective retirement accounts (and HSA) are far superior to 529s (and we could barely max those retirement/HSA accounts, so they are the priority). The 529 plans really just have some state tax benefits and tax free growth, though these can be clawed back, with penalties too, if not used for qualified expenses. So really 529s are not that great, and I almost regret using them, especially since at the time we weren't maxing all retirement accounts. Still, at least we have these savings for a designated purpose, and it provides part of a budget.