From my testing with solar garden lights, they don’t appear to have any charge-termination circuitry. They simply charge at about 100-200mA in full sun, and when the battery is full it just keeps on trickle-charging. Some batteries handle that better than others, and that’s probably why they use low-capacity cells in solar lights.
But if you’re only reading 0.5v from your solar panel in full sun, I don’t see how it could charge your batteries. That sounds like most of the solar cells in the panel are damaged.
You could charge the batteries separately, and install them fresh every evening, but that’s probably not the kind of effort you want to do. I think your best action is to buy new lights.