I would be very impressed indeed if you can successfully predict in which areas technnological advances will be made, before they are made.
Technology aside, there is no limit to the energy that can be had from the sun.
As you say there is a maximum rate of photons per square metre which can be converted to electrical power.
So to increase the amount of power output, all that is required is to increase the amount of area exposed to the sun. Some very simple calculations confirm we could easily meet current whole world power demands by covering a modest amount of the deserts of the world in solar panels, even using current technology. The hardware needed can be manufactured from the energy generated by an initial bootstrap quantity of panels.
I cannot see the limit that you appear to see.
Nuclear energy to date has been a hiding to nothing. In no cases has a nuclear power plant ever been decommissioned to a non-hazardous state. This is for entirely economic reasons. Thirty years after the monies from commissioning are long gone in the pockets of those retired on the proceeds, no-one cares to spend the money needed to make them safe, as there is no profit in it, so, as things are (driven by profit) they are destined to remain in an increasingly hazardous state for thousands of years to come.
As such nuclear energy is actually a very bad investment overall. There is no positive return on investment in the longer term. All that results is yet another form of deadly planetary pollution.