Oh I think I get it.
You want a USB 3.0 hub that has only USB 2.0 ports for the peripherals so that all the bandwidth is used for HDMI and not the peripherals.
I could be wrong, but I'm pretty sure USB 3.x never supported 4k at 120hz with HDMI. Only thunderbolt could achieve this nativley.
There is a way with DP alt mode which worked using some bonus display port protocol magic which is why the suggestion is to use a DP to HDMI adapter.
Actually I'm pretty sure even DP requires at least 3.2 but I would need to check.
HDMI 4k @120Hz was added in USB 4.0 which won't work if your phone is only 3.x.
You need a hub that supports DP alt mode so that it won't downgrade the output: https://www.reddit.com/r/UsbCHardware/comments/1dahjah/psa_usbc_bus_powered_dock_with_dp_14_alt_mode/
I think yout phone needs to support it too though, otherwise it won't work.