In a nutshell:
COBRA is an option where you can pay your former employer to stay on one of their health plans for a limited time. This will be unsubsidized. By electing COBRA, at least through the end of the year in which you retire, you won't need to reset your deductible mid-year. If you already have a pretty high income from that first partial year of work you might not qualify for any ACA subsidies anyway so that's another point in favor of considering COBRA. The ACA coverage may look like a better deal beyond that first year if your income will be on the low side in retirement (but maybe it won't be if your husband is still working?).
Be aware that if you do switch from COBRA to ACA you should probably plan to do it during the year-end open enrollment. You can only sign up for ACA coverage at open enrollment or during a qualifying life event. COBRA eligibility expiring does count as a qualifying event, but voluntarily leaving COBRA does not.
ACA subsidies come in two main flavors. The first is premium assistance, on a sliding scale based on your income. Put your family members, ages, and expected income into your state's ACA exchange website and it should give you a good idea of how much this will be. The second type of subsidy is for out-of-pocket costs. If your income is below 250% of the poverty line you can qualify for these. These show up in the form of lower deductibles/copays/etc. on silver plans only. Again, if you put in an income in this range the exchange should show you the modified options. The 200-250% of poverty line options will be a pretty minor difference, while if you get a bit below 200% it's a really noticeable change.
Get into very low income territory (<138% of the poverty line) and you may be offered Medicaid coverage. This can actually be a pretty nice program (no premiums, no out-of-pocket costs for any covered service), but it comes with the caveat that you can generally only get care from the in-network providers. If you're offered Medicaid you can't get subsidies from the exchange, so if your income is at all near the line and you prefer to stick with the private coverage through the Marketplace, make sure that your income each month is above the line. Medicaid uses a month-by-month income standard, while the ACA premium subsidies look at your calendar-year income. It's weird.
It's common for people looking at ACA options for the first time to be dismayed at how high the premiums and out-of-pocket costs are in comparison to their prior employer insurance. This is because health care is very expensive here in America, but your employer was picking up a big portion of the cost. Now it's all on you (after any income-based subsidies).
If your husband's business is a sole proprietorship (or an LLC taxed like one) you will be eligible to deduct your ACA premiums from your income under the self-employed health insurance deduction. I believe COBRA will not qualify for this.
The good news to all this is that health care really doesn't need to be this giant roadblock to retirement. You will have options. It's just an expense you need to plan for, like any other.
