In addition to @aj485’s excellent analysis…
Let me add that DH and I did use ACA insurance before we aged into Medicare. This worked well for several years.
However, DH is a year older than I am. When he became eligible for Medicare I was blindsided by a huge tax bill. The IRS imputed our entire income to me that year which meant that we were no longer eligible for the ACA. I had to pay back the entire year’s ACA subsidy in an unexpected lump sum.
Wendy