Huh?
The vulnerable are already gone, or vaccinated now. The young and healthy are the ones moving the thing around today. They mostly don't get tested because they have no symptoms.
As of Dec 2020, the estimate you posted says 83.1M were infected in the US, counting both reported and unreported cases, a ratio of 4.6. That should be about 18.2M reported, which is a little lower than the worldometers number of about 20.6M, probable due to CDC data lag.
You then said you think we're up to 215M now.
Worldometers says we're at 32.8M and their number was about 20.6M on Dec 31, so keeping apples to apples they count about 12.2M new cases in the last 4 months.
*IF* you meant to say that the current ratio of unreported to reported was around 6.5, then the right thing to do would be to multiply 6.5 by 12.2M, which gets you to 79.3M, and add it to the previous 83.1M, for a total of ~160M. I don't accept that the current ratio is 6.5 w/o evidence, but I'd stipulate that it's at least a plausible number. That would bring the OVERALL ratio, counting both the separate Mar-Dec and Jan-Apr, only to about 4.95.
But that's not what you said. You said that the overall ratio has now jumped to 6.5, which means that the ratio between Jan 1 and now must be MUCH higher than 6.5. To get to 215M, that means that the ratio between Jan 1 and today is actually 10.8 unreported to reported cases. I don't see any evidence or explanation to see a jump that high... And then that there'd be >130M new infections over the span of 4 months, without anyone noticing. Doesn't pass the smell test, dude...
I realize that you think everyone has had this thing, twice. As such, you've
consistently taken the highest possible estimates for the ratio of unreported to reported cases. I get that. I'm just saying that the numbers you're getting to are now well into the implausible range.