Thank you very much @xlm567.
Yes, the passive heatsinks are enough to prevent them from overheating.
Only the XU4 throttles a bit.
On the XU4 I had also my wallet at the beginning, but for me the wallet was much faster and reliable on a real x86 computer and so I moved it.
Combined these 5 SBCs generate around 1 GRC/day.
But I didn't optimize the project choice to maximum profitability.