Is a Dark Rock 3 enough for a Ryzen 7 5800X, or should I switch to a top-mounted AIO?

The important airflow detail is whether the CPU cooler is getting reasonably cool air instead of mostly recirculated GPU exhaust. With a front-to-back airflow path, the Dark Rock 3 should be fine. A top exhaust radiator is a sensible layout if you choose an AIO, but it is not necessary just to protect GPU temperatures. Avoid mounting a radiator at the bottom, since that can let air collect around the pump or water block.

The Dark Rock 3 should be capable of handling a 5800X. It has a substantial heatsink and six heat pipes, so I would not replace it automatically. Check your actual load temperatures and fan noise first. If the CPU stays within a temperature range you are comfortable with and the fan is not annoying, there is little reason to upgrade.

An upgrade makes the most sense if the current cooler is louder than you want. A newer dual-tower air cooler, such as a Phantom Spirit 120, would likely offer an excellent improvement for much less money and with less pump-related noise or failure risk than an AIO. Older Be Quiet! coolers can also perform less effectively on AMD's offset chiplet layout, so mounting and thermal paste coverage matter.

A top-mounted 240 mm AIO exhausting air may reduce heat buildup in the case slightly, but it probably will not produce a noticeable GPU temperature improvement. Your three front intakes already supply fresh air, and the lower intakes should feed the graphics card directly. Expect any GPU difference from changing the CPU cooler to be small, often only a degree or two, assuming the case also has a good rear exhaust fan.