What I'm hearing is that you spent a year designing a more complicated proof-of-work challenge rather than moving to a challenge system that actually impacts scrapers. Consider revisiting this decision!
At this point isn't it basically auth-only?
Rant: (genuinely wondering too, and RFC, request for conversation) at this point don't we have Google, etc. basically doing Real World ID Verification, but without an open protocol backing, using it to corral users into their ecosystem and gather data, and leaving us without some open and distributed alternative like the (fairly well working it seems to me) Public Key infrastructure? (PKI)
I know Sam Altman (with Proof of Human or w/e) and others have suggested and also have their own alternative (which I think is basically what Google already does), but I think this should simply be built based on an open protocol and then hopefully not-too-large (and with a healthy competition) local companies could do it for you.
Fixing a malware infection should be like fixing up anything else broken around your house (clean things up, re-auth yourself with the ID verifier, done).
Also, bots cannot simply automate renewing the credentials of course, after expiry you should be required to physically (or in a trustworthy digital medium) re-authenticate yourself preventing any kind of mass attack, you'd need millions of people to perform authentication multiple times a day (by that point it seems safe to assume they'd rather fix the malware source in their personal devices).