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I don't think bandwidth is reserved. Thunderbolt3/USB4 is a 40Gbit link which can also carry display data. The USB-C stuff with dedicated lanes for Displayport is ignored then. There's no EGPU support on the M series, but even if you could you can just use the GPU output so the entire Thunderbolt bandwidth is dedicated to the PCI-E card.

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> I don't think bandwidth is reserved

It is reserved.

"Thunderbolt 4 has a maximum bandwidth of 40 Gb/s. However, this is a bit misleading because not all of that bandwidth can be used for data transfer. Approximately 8 Gbp/s can only be used for video, leaving 32 Gb/s for non-video data (PCIe 3.0: 4 lanes x 8 Gb/s). Further accounting for PCIe 8b/10b encoding and TB4's own overhead, you arrive at a peak data throughput of approximately 22 Gb/s. Not 40 Gb/s but not slow either."

Source: https://tripplite.eaton.com/products/thunderbolt-4

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