Zen 2 and Zen 3 processors don't actually need a whole lot of cooling power. The issue isn't that they produce a large quantity of heat, but rather the 7nm process means there is not much surface area to transfer heat out of the die into the heat spreader and then the heatsink. The processor will run hot either way just because it's hitting a heat transfer bottleneck. As a result, having a large radiator is not really necessary. Even the worst 240 AIO in existence should be more than sufficient unless you're doing serious overclocking with unsafe voltages. A 360, let alone 420, will be totally overkill.
The advantage of an AIO vs a large heatsink like the NH-D15 is mostly that the heatsink is very large and bulky and gets in the way. The cooling performance will not be that different--maybe a few degrees either way. If the size doesn't bother you., then personally I'd just stick with what you have.