Are there any benefits at all in listing my wife, who already owns a condo unit, as an “Occupier” under the resale flat I just bought?

Spouse as an occupier has various effects, off the top of my head it may affect her when she decides to buy and/or sell her condo.

Best is to email HDB for this rather than a property agent to get a firm/conclusive answer.

However, as a caution, maybe can consider getting an LPA done with your wife as your donee, in the event anything happens to you, she can make decisions on your behalf like selling the flat to raise money for medical treatment.

Since I'm not sure when you got married vs when you purchased the resale flat under the Singles scheme, this is a tricky question to answer :)

By right though, your spouse must not have any interest in private property if she's listed as an occupier in an HDB flat pre-MOP. (Past that Minimum Occupation Period, it's fine.)

The only real benefit is that if you pass away and your wife is listed as an occupier, the flat will legally pass on to her (provided she satisfies HDB's eligibility rules for owning a resale flat). It gives her a small measure of emotional assurance that she'll be taken care of, I suppose -- though she definitely wouldn't be homeless given her current property ownership.

I think the best option right now would be not to list her for now -- and then revisit the question after you've fulfilled the MOP!