Start with, "I'd like to be converted to full remote, what's the process for that?" The answer is probably "I don't know but I don't think it would fly." Insist that your manager ask up the chain for an official answer.
If the answer is "nope", next say: "I'm not comfortable returning to the office for health reasons. Will I be fired?" This is two-pronged:
a) it introduces private healthcare decisions, a legally fraught topic they do not want to deal with. Is it legal to fire someone for refusing to return to the office despite health concerns? Probably, but no one knows for sure and your employer does not want to be the test case. (If they ask, "what health concerns?" remind them that's private. Don't invent a fictitious medically-fragile aunt.)
b) It may force them to confront the fact that they will lose people over this. They very likely think of wfh as a perk, something they can decide by fiat without consequence. If no one quits over it, that's exactly what it will be.
If they stick to their guns, it's time to look for another job I suppose. If they have some explanation for why wfh is bad ("productivity is down, look at this graph", or "the value of hallway conversations" or whatever) I wouldn't bother arguing unless it's a small enough company that you can argue with the person who made the decision.