Whenever you lose coverage you can go on ACA coverage so no, that does not matter.
while this is true, quitting mid year often means your annual income will be too high to qualify for ACA subsidies. anecdotally, seems like most people in this instance go on COBRA since the cost is similar to non-sub ACA, and they've already met (or are part way to meeting) their deductible.
I do have a fairly high salary, so I assume it would be better to quit as early in a given year as possible.
When we went, we retired early in the year and dumped as much into our 401ks and HSA as possible before jumping (end of Q1, after bonus hit). DHs paychecks were under $100 near the end. HR actually called him to double check his contribution percent, just to make sure it wasn't in error. I couldn't get mine as low, since my company wouldn't allow more than 40%. So sad.
But it kept our income low enough to get the ACA subsidies we wanted for the year.
As for your questions, my 2 cents:
1. Bonds are rough right now. We have found that using a money market (we use vanguards settlement fund) gives us about the same without locking us into a set time frame like bonds do. We like the liquidity, its our efund. There are also some high interest checking accounts out there, you can use those too.
2. We have cash-like for our bare bones expenses for ~two years (settlement fund mentioned above, and some in other checking/savings). We picked two years since most downturns last ~18 months. Our barebones are very barebones, but they cover things like mortgage, student loan, out of pocket max for insurance, insurance premiums, and sundries of survival. If the market goes down, we will have to take out some money, we know this because we will still have to reach our ACA minimum amounts or we get shunted over to medicaid (or have to pay back to subsidies). But it will keep up from take out much in a down market, and lets us be picky as we have various investment (not all VSTAX like you have).
3. See above first part.
