With PCIe or Thunderbolt or USB or DisplayPort, bandwidth is not divided into each branch. Bandwidth is shared. If no data is being transmitted to any other device then a device can get near max bandwidth for itself (up to the minimum of the max allowed by all parent devices).So you say pci bandwith is allocated whether there's a TS4 hub or not? I was under the impression that a TB4 hub (which is also in the TS4) splits the full PCIe 3x4 anytime. Apologies if I misunderstood that badly. My thoughts on that 2,5GB port still hold up.
For example, you can connect any number of USB 10 Gbps devices using 10 Gbps USB hubs. All the devices will be able to do ≈10 Gbps if no other device is using any bandwidth at the same time. There is a slight drop for each hub in the chain but it's only a fraction of the 10 Gbps.
2.5 GbE is better than 1 GbE. A 10 GbE PCIe controller would be limited to 8 Gbps in the TS4 since there's only one PCIe gen 3 lane to use. I suppose in this case a 10 GbE USB adapter could be slightly faster but I would avoid USB Ethernet adapters if PCIe is an option. Another option would be for the TS4 to include a Thunderbolt 3 controller attached to the Thunderbolt 4 controller but that adds cost. There are some eGPU enclosures that contain two Thunderbolt 3 controllers to improve USB performance. This indicates that a single Thunderbolt 3 controller has a design flaw regarding doing both USB and PCIe gen 3 x4 at the same time, at least when the PCIe gen 3 x4 link is connected to a GPU. The most flexible option is to connect a separate Thunderbolt 3 10 GbE device.