As the others noted, SERMA is granted if you retire (and are not terminated for cause).
If you are not working, then depending on what state you are in, you may be pushed into a (no cost) state medicaid plan instead of the healthcare market place.
KNOW the income thresholds when applying to the marketplace, or risk being pushed to a state medicaid plan (you may not like that level of coverage). Consider talking to a marketplace specialist about this.
If your income is less than 38k (less than 400% of the federal poverty level), then you can get a high deductible blue cross plan for $100-$200 per month, after subsidies.
If you estimate income which is below the state medicaid threshold (around $20k, depending on the state), then you will become ineligible for the marketplace, and will be pushed to go on the state medicaid program (or buy IRMP or some other non-subsidized insurance plan).
IRMP is more expensive because it has a low deductible. If it is $1500/mo, then the high deductible blue cross is $1200/mo, with about $1100 of that subsidized.
The marketplace requires you to opt-out of SERMA, to be eligible for subsidies. The state medicaid does not care about SERMA. It is based solely on income being less than the threshold.
You have the option to op out of SERMA each year (by Dec 15th for the next year).
If you are on COBRA, then a month or so before that ends, you can apply for the healthcare marketplace, start up IRMP or whatever change you want for continued coverage. Exiting COBRA is considered a qualifying event by the marketplace, enabling you to start coverage once COBRA ends (and not have to wait till the next year).