Several of your numbers don't make sense to me.
First, ACA subsidies are available up to 400% of FPL (and even higher through 2025), and the FAFSA exempt from asset reporting limit is 175% of FPL for MFJ. I don't see how your 400% FPL number can be $75K and your 175% of FPL number can be $65K.
Second, I don't understand how you can have unrealized gains of $90K and then refer to a "huge tax hit". I read it as having unrealized gains in your taxable account of $90K, and if that's the case, CG on $90K would not strike me as a "huge tax hit" in the world of someone who sounds like a multi-millionaire.
Third, I'd double check your FPL numbers themselves. Figure out your family size (5?), where you live (CONUS?), and what year FPL numbers to use (2024?). Note that the FPL breakpoints for FAFSA purposes and the FPL breakpoints for ACA purposes may differ by a year due to the different timing requirements for the two programs.
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On the strategy front, I'd suggest overall a different strategy: have your wife retire today, end up with whatever tax return you end up with for 2023, and then file a FAFSA appeal for professional judgment citing the retirement and your lower (presumably) 2024 income.
You haven't given hard numbers, but it sounds to me like the taxes you would pay by cashing out your taxable investments, plus the growth that you would have had on those taxes you're thinking of paying - compounded over the next five to seven years, plus whatever loss in investment gains you're creating by deliberately lowering your investment returns - again compounded over the next five to seven years would possibly exceed any FAFSA benefit you would engineer. At least try to run the numbers before doing your plan.
As another strategy thought, since you mention the UC system I'm assuming you're in California. If you have three kids who want to go to college and you've either not prepared for that or are not willing to use your assets to pay for their college, then I'd strongly suggest essentially informing them of this and suggesting they consider the community college route for the first two years, then transfer to one of the in state schools. From what I've read over the years, California has put some good guardrails in place to help ensure that kids who do two years in CC and finish at a state school will stay on track to graduate. I've also heard that CCs in CA are a lot cheaper than the state schools.
You can also start, although it's a bit late for some, on all the other strategies to save on college - careful school choice, careful choice of majors/degree programs, scholarships, grants, tax credits like AOTC/LLC, AP/CLEP/IB/concurrent credits, part time work, gap year plus full time work, WUE, 529 state tax credits, being an RA, etc.