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They've built those in the lab decades ago [0]. I don't know what the large drawback is that prevents them from being more widely used.

In the thermophotovoltaic case they were looking at emitter temperatures between 1,200 and 1,350 K.

[0] https://sci-hub.se/https://doi.org/10.1063/1.1867178 ("Thermophotovoltaic Converter Performance for Radioisotope Power Systems", 2005)

Blackbody radiation is the wrong approach. If you are going to slap a PV panel onto a radioactive source then tune it for the emitted radiation rather than the blackbody heat. Radiation from nuclear decay should be very narrowband, allowing for a more efficient PV solution.

There is no emitted radiation: the heat source self-absorbs its own alpha radiation and converts it into heat.

Arrayed as a thin film on a substrate, you can use the kinetic energy of the alpha particles directly for thrust. The substrate will need to dissipate some heat from nuclei emitted in the wrong direction.

What is the status of making power directly from e.g. alpha radiation?

You could also do https://en.wikipedia.org/wiki/Betavoltaic_device