Just some thoughts / considerations about your question;
Looks like you want a live encoding with a webcam, right? 720p 60fps is familiar with webcam, i guess.
I have rk3399 with a dual camera (ov4689), although the camera can work at 720p 180fps (according to specification) , the available driver works with 30fps, unless it is modified by a kernel expert. You may face the same problem with imx219, i don’t have one to confirm this is true. There is a post about imx219 with rockpi 4, you may ask him what frame rate is possible.
I have not done any benchmark, rk3568 seems to have a better and faster encoder than rk3399 but latency does not depend only on encoding speed, and rk3399 is a more powerhouse that could influence the final latency results.
If you set it for 720p 30fps it could be possible to do some simple benchmarks and compare.
BTW, Rock 5B will have imx415 (4K@90fps), but again you may face something like this: IMX415 different sensor modes (60+fps) · Issue #245 · rockchip-linux/kernel · GitHub
But it is just a matter of time till someone fixes the driver.