High frequency relay options for 5V 20A DC
If you really need to switch that amount of power at that high of a frequency, you likely need to use GaN or SiC transistors. Be aware that this represents and extremely challenging engineering task, and that you could likely publish a paper on just the switching circuit alone.
If you can afford to switch at a lower frequency (say, 10-100kHz range) a half bridge MOSFET circuit is likely more than sufficient for your application, especially one which has a separately driven internal charge pump to allow for 100% duty cycle PWM.