Ryzen isn't Intel. And applying well known Intel variables such as temps to a Ryzen cpu don't work.
At idle, Intel starts dropping all the voltages and speeds simultaneously across all cores, but all cores remain active, even in a low power mode, so temps across the entire cpu are similar at a barely above ambient case temp state. Any windows/background tasks are spread across all the cores and see periodic spikes that can exceed 50ish °C.
At idle, Ryzens shutdown parts of or entire cores, leaving only one core doing all the background/windows work. So one core sees the entire workload and stays at 40-50ish (sometimes higher depending on cpu and voltages and loads) °C until power states progress to a lower state and stop the background tasks etc.
So your idle temps on a Ryzen are perfectly normal, representing just 1 core use, not the entire cpu use.
A better cooler won't do much for idle temps, power usage is well below even a modest coolers ability. It's load temps where power usage challenges cooler ability and efficiency where a better cooler is most helpful.
It's not the width being the issue, 140mm already fits on top, however there's reservoirs on the top and bottom of the rad, the typical 240mm is @ 275mm long, the average 280mm is over 320mm long. A rather large difference from 2x 120mm fans side by side.
So a 240mm fits, a 280mm is too long. Even if 2x 140mm fans will fit, physically 5mm longer than a 240mm AIO. If that much..