I am now reassigning a new buffer for the SAI ISR to fill in once I send each packet by using FreeRTOS_GetUDPPayloadBuffer and it is working fine. I would say I have a solution working but I think it would be good to document somewhere (maybe I’ve overseen it) that FreeRTOS_GetUDPPayloadBuffer just works for 1 packet.
I’ve seen at least one post in the past from somebody who was using as well FreeRTOS_GetUDPPayloadBuffer to preassign buffers, so the info will be surely help for future developments.
It would be of course great to find a solution to keep the FreeRTOS_GetUDPPayloadBuffer buffers reserved without impact in performance until they are explicitly released by the application. This would spare me one call to FreeRTOS_GetUDPPayloadBuffer per packet and the related speed impact (my STM32H7 is at its limits 
).
Thank again for your support!, I am really happy with FreeRTOS+TCP, it is perfectly stable (in comparison to LWIP which was very unreliable and stopping working after a few dozen hours) and very fast