Jokes/notjoking aside, how long are you planning to work? If you're getting close to FI and stock prices scare you, read Early Retirement Now and prepare a bond tent or something so you are protected in your overall portfolio, not just this year's purchases. If you're going to work another decade, low prices just mean the stocks you're buying finally went on sale, what's the problem?
https://earlyretirementnow.com/
If the 4% rule holds up, I am at FI now. I am personally targetting a 2% SWR which I am not at yet (specifically my goal is to get to FI on dividend income alone). This will give me added safety/peace of mind, and also room introduce a little more room for lifestyle inflation - I have young kids and am unsure how much more expensive they will get in the future. With average 6% market returns I can be there in a few years if I keep working. My job is also in healthcare and to be honest I would not feel right leaving now with everything going on.
Ah, I see. In the big picture, you have cost uncertainty due to children. Plus, emotionally, you want nice solid safety cushion. Very understandable.
Glad you're taking other factors into account re continuing to work - seems to me like an important step towards well rounded self determination. You sound like you're likely to be financially independent from now until the day you die, so learning how to live on nonfinancial motivations is a big deal and I congratulate you. Anyway, when it comes to details like whether to feel guilty leaving work, whether to accept 4% as the ultimate line making you safe, etc. - you are the boss. Other people will comment legitimately about these issues because getting people over their fears is a big and valuable part of this forum's culture, but your decision is the important one. Fwiw, IMHO both kids and temporary skepticism about 4% at current prices are perfectly reasonable, therefore a safety margin is reasonable too. Full respect from me.
With the above paragraph as context, it's a judgment call whether in your overall portfolio you should seek more bonds/cash, other diversification, or simply more stock. In my view, still the important thing is to make the monthly/ annual contributions fit the bigger picture, fulfilling whatever overall allocation you think is best. Compared to the simple fact of reaching investments = 25x expenses, the allocation of this year's income is minor - not just in the sense that it's a small part of the whole, but also in that diversification has safety value, so any choice is reasonably effective in protecting you! Forgive us the portfolio analysis - context is important in trying to find the best answer to your question.
So here's an answer!! :)
VMFXX offers extra stability by focusing on US govt securities, but seeks to provide income too. If I understand correctly, because this is a "money market" ETF, its returns are low but safety (lack of volatility) high because it focuses on very short term securities. I think that's as close to "no market fluctuations" as you're going to get in timeframes of less than a year.
https://personal.vanguard.com/us/funds/snapshot?TTVETF=VETFCNTRL&FundId=0033&FundIntExt=INTUnfortunately, I think what you'll find is that in today's low interest rate environment, there isn't much interest available in any passive investments like ETFs unless you are willing to accept the risk of market fluctuations.
If you want to put in some effort, there are threads by people who use their cash to open new bank accounts at banks that offer signup bonuses. The effect is similar to getting high temporary interest. Risk of market fluctuation is zero! You do have to beware of account minimums later, and generally conform to the terms of the accounts, but if you're careful, you can open (and if appropriate close) accounts in just a few months and keep the bonus without paying any fees. You can't do an infinite number of these, but a few is easy money if you're comfortable following up on details, apparently. That's as close to a free lunch (riskless interest) that I know of.
https://forum.mrmoneymustache.com/share-your-badassity/bank-account-churning-how-to-make-$1600-in-a-year-by-being-organized/800/
https://forum.mrmoneymustache.com/reader-recommendations/two-easy-bank-bonuses/Fwiw, I think a lot of the reason you're getting so many comments on broader issues is that you're asking for something - riskless interest - that doesn't exist. In good faith, respondents are assuming that behind that possibly uninformed request, you have legitimate needs and we want to help you meet them.
Best wishes in the future - it sounds like yours is quite bright! And congrats on technically reaching FI. :)