I would say I wouldn't bother with the 10Gb Ethernet in your situation (or for the majority of people out there). Based off what you said, you wouldn't ever be able to take advantage of the faster network speeds, and it would basically be a waste of an investment. Keep in mind that to take advantage of 10Gb Ethernet, it requires more than just the port on your iMac. You would need other networking equipment, such as a 10Gb router, and also a reason to use it.
As for future proofing, if you ever find yourself in need of a faster Ethernet port, you could always just get a TB3 10G Ethernet adapter.
This kind of reminds me of CAT5e versus CAT6 back in the Mid 2000's. 15-ish years ago, home builders were pushing the more expensive CAT6 over CAT5e for new home construction due to 'future proofing', saying that in a few years CAT5e would be too slow for average home network traffic.
Fast forward to 2020, still hardly any consumer-grade devices have 10Gb Ethernet.
Data transfer within the network would probably be the most common. Less common would be for 10Gb internet speeds, which is available in some places of the US, but is not commonly used.