Are you running the latest update patches for Sonoma (v. 14.5)? If not, then install the 14.5 update & test again.
Disconnect all external devices in case one of them is causing a problem or a compatibility issue with one another. Test by connecting just a single drive. Test with each USB port including rotating the USB-C connector of the cable 180 degrees upside down.....meaning you need to make eight tests with each drive (2 for each USB-C port on the laptop).
Try booting into Safe Mode to see how the drives perform.
How fast are the transfers over Ethernet if you only use a single Ethernet adapter? By using two Ethernet adapters you will be masking the issue you are experiencing with the USB-C ports when connecting various drives.
How much Free storage space do you have on the internal SSD (ignore the "Available" value since it is very misleading)? If you have 80GB+ of Free space on the internal SSD, then you can create a new APFS volume (make sure to give it a unique name) followed by installing macOS onto it. Test how the USB-C ports behave while booted into the new OS. Do not install any third party software, do not restore from a backup, and do not log into your AppleID or iCloud. Tests must be performed with a clean OS as shipped from Apple. Before deleting the new APFS volume, make sure the default Startup Disk in System Settings is configured for your main OS you wish to keep in order to minimize any issues when deleting the APFS volume after testing.
You can also try performing a DFU firmware Revive which resets the T2 security chip & system firmware. I doubt this will make a difference, but it never hurts to try. Although this should not affect the data on the internal SSD, I highly recommend you have a good backup first just to be safe. If the Revive process does not complete successfully, then it could leave the data on the internal SSD inaccessible.
FYI, USB sticks are generally very slow even if they are labeled as USB3 so they are not good test cases. Plus the quality of USB sticks is extremely poor. A USB3 SSD is best, but even some SSDs can be very slow especially when writing to them for more than about 30-60 seconds. Testing with an SSD is better done by transferring data (reading) from the external SSD & writing to the internal SSD since the Apple internal SSD is of much better quality than many third party SSDs today. Testing this way will better reveal the actual speeds of the USB-C ports since the external SSDs will be slower than the internal SSD due to the limitations of the USB-C ports on this laptop which are limited to 10Gb/s (aka 1GB/s transfers). Also once an SSD is throttled by writing too much data in a short period of time, it can take the SSD a while to recover (sometimes days).
