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It is worth noting that most of these lights (aside from the DRLs) will spend the majority of the time turned off. Thermal management may not be required for them at all.
Given our current dimension restrictions, this is the best that an off-the-shelf heat sink can do: https://www.digikey.ca/en/products/detail/assmann-wsw-components/V5619A/3511388
Fortunately according to the above calculations, a heat sink is not mandatory, but it may still be useful. It helps that the heat sink from the link above is very cheap so we can afford to experiment with it.