wired connection speed slower than plan speed

... I am using the TP-Link TC7650 ... I am paying for 1000MBbps ...

The device is not rated for Gigabit service. See the "All Compatible Devices" link near the bottom of https://www.xfinity.com/support/devices (at the moment it points to https://assets.xfinity.com/assets/dotcom/projects/cix-4997_compatible-devices/2024.04.03%20Full%20List%20of%20Compatible%20Devices.pdf, but this changes from time to time). There, Comcast rates the TC7650 for speeds "Up to 840 Mbps". Using devices on Internet speed tiers for which they are not approved tends to produce unexpected results, often speeds well below the ones you are paying for. 

Consider picking up one of the devices that is approved for Gigabit service, or dropping back to a lower tier.

Realize that although Comcast calls some devices "Approved" and some "Recommended", for all but a few devices those designations only mean "approved/recommended for use with their Internet service", and not necessarily approved for your plan speed.

I would like 1 gig but I want to be sure I will get that before spending the money on a new modem. Why do you think I am getting around 600Mbps with a max of 700Mbps with a modem that states it is 1000 Mbps and Xfinity states is 840 Mbps? Shouldn't I at least be getting closer to 840 Mbps, not 600 Mbps average? What proof do I have that upgrading to an ARRIS Surfboard S33 for example with 2.5 Gbps I will actually get 1000 Mpbs?

The unit is a 1Gigabit device, why does it only go up to 840 Mbps on Xfinity?

I don't know. Comcast has never said. But the speeds manufacturers claim are typically theoretical or lab-bench speeds you'll never see in the real world.

Also if it is 840 Mbps why am I seeing a max of 700 usually closer to 600?

We tend to think that if Comcast rates a device for a given speed, it should work at that speed if used on a higher speed tier, but that's not what happens. Unapproved devices always run slower than Comcast's rating. Again, they've never explained how they provision unapproved devices, so we don't know why the speeds are lower.

... What proof do I have that upgrading to an ARRIS Surfboard S33 for example with 2.5 Gbps I will actually get 1000 Mpbs?

None. All you have is the statement the only devices approved for Gigabit service will deliver Gigabit service, and that devices not approved will not.

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I replaced my TP-Link modem with the ARRIS Surfboard S33 and saw no speed improvement. Xfinity it would be great if you would chime in here and suggest what the issue might be.