The nice thing about the IRS is that intent is pretty irrelevant. You make money, you don't pay taxes, your wages get garnished and your bank accounts seized for taxes, fees, penalties and interest. There's no messing around with the IRS, intent not required. If they can show intent you go to prison.
[edit] The difference with under the table work is that you get cash, then you spend the cash and not a record was made, then you've got the IRS forensic accountants on your ass if you do it too much. With crypto if you want to get your purchasing power back you pretty much have to go through a fiat gateway, every transaction there is logged, and likely reported.
The first time I had to deal with them, I was surprised that the IRS is very professional, patient and courteous to me! I believe if they assume you're not trying to cheat taxes, they will help you to solve your case.
I can't stress how common this is with cash only side gigs. People hope they don't get caught and if they get caught they play dumb and agree to pay X per month to settle the bill.
They need to show intent or get you to lie to them if they want to put you in prison (which they don't because they want your money instead).
The typical MO for tax evasion is to just not report your side gig (crypto or otherwise), hope you don't get caught and play dumb and pay up if you do get caught. This doesn't change whether it's crypto or doing under the table work which is the point I'm trying to make here. You either pay up front the right way or have a chance of not paying at all (tax evasion) but if you get caught doing that you pay slightly more for not paying up front. Anyone who is being charged with tax evasion is doing something very stupid.
It should be obvious but the people who've revived these letter should play dumb (if applicable) and pay up at this point since clearly the IRS knows that they owe more than they've paid.