Over heating issue

Pic unrelated

So it does fine when parked/idling (even cools down if it’s hot). But starts to overheat over 40 mph. Under that it’ll be fine with the heat on as well.

Unfortunately, I think this means it’s not the thermostat, although I could be wrong. Since I don’t think it would cool down at all with the thermostat stuck.

Waiting for it to cool down before checking coolant level (overflow tank is too dirty to check) but there’s no obvious leaks, so I’m expecting the coolant level to be fine.

Also oil color is fine and no white smoke coming out tailpipe. Water pump was replaced before I got it.

Does this mean if the coolant level is fine, I need a new radiator?