Plate tectonics of that style are a transient thing. Let's take earth at an example, where they started some way through the Archean eon, and are expected to come to an end up to 1 billion years from now because the mantle has cooled down so that subducting oceanic crust will get stuck.

On Mars, oth, this style of tectonics have never fully developed and tectonic activity came to an end much earlier in its history, or let's say took a very different course.

A planet of your invisioned age of 8.4 billion years and in the size range would long have cooled down and it's surface would have had ~3 billion years to erode and become less featured than that of a planet with active plate tectonics.

But, in principle and if you just wave the energy flux away (it's world building after all), a "supercontinent cycle" these days on earth takes somewere between 300 and 500 million years. That's roughly the time span for continents to breaks up, drift apart, and close again. This is called a "Wilson cycle", driven by density differences of cooling oceanic crust, and up to 3 of these would fit into your time span under current earthly conditions, making the envisioned changes principally plausible to me.

Plate tectonics of that style are a transient thing. Let's take earth at an example, where they started some way through the Archean eon, and are expected to come to an end up to 1 billion years from now because the mantle has cooled down so that subducting oceanic crust will get stuck. Continents are more complicated since they float on the denser mantle material, erode, fold up, are lifted and subside and so on.

On Mars, oth, this style of tectonics have never fully developed and tectonic activity came to an end much earlier in its history, or let's say took a very different course. Marsian surface can be bilions of years old, while oceanic crust on earth doesn't get older than ~200million years.

A planet of your envisioned age of 8.4 billion years and in the size range would long have cooled down and it's surface would have had ~3 billion years to erode and become less featured than that of a planet with active plate tectonics.

But, in principle and if you just wave the energy flux away (it's world building after all), a "supercontinent cycle" these days on earth takes somewere between 300 and 500 million years. That's roughly the time span for continents to breaks up, drift apart, and close again. This is called a "Wilson cycle", driven by density differences of cooling oceanic crust, and up to 3 of these would fit into your time span under current earthly conditions, making the envisioned changes principally plausible to me.