It’s not the dock it is the cable and the USB-C spec.
USB-C never was designed for long distance data transmission. That is what ethernet etc. are for.
USB-C works great with high speeds at short distances (direct plug, dongle, short cables).
HDMI 4K @ 60Hz via P3 (10bit per color, if the display can handle it) would result in 11.14Gbps being required (at peak, without compression) as a bitrate. You can most likely assume 4-5Gbps being used under real every day circumstances alone by the display.
Any USB drive connection will add to that load. Usual read speed of a HDD is at 100-120MBps so around 0.8-1Gbps. Two of those that are also writing from time to time.
The keyboard doesn’t take up a lot of bandwidth, but requires a stable connection.
USB-C devices will negotiate upon connecting and negotiate about bandwidth on a low system level to prevent data loss. That is most likely why your display is only offering a lowered resolution. (Check the resolution in your system preferences or the display’s on-screen info pane.)
Most USB-C extensions are only rated for 5 or very rarely 10Gbps and they are still comparably short. We are years and probably another standard shift (or two) away from the “one plug” docking station replacement that allows daisy chaining all our devices that we all dream of.
You could get away with two extensions to two hubs, though.
One for the hard drives and one strictly for the display.
I myself switched to a 1.8m USB-C extension (rated 5Gbps) with up to 100W power delivery. I plugged my MacBook’s charger into the “power delivery”-in of my USB-dongle hidden under my desk. I mainly use the hub to read of SDcards from my camera or occasionally off of a pen drive and of course to have a reliable Gigabit LAN connection to my NAS.
Additionally I use a USB-C to DisplayPort for my 4k display. DisplayPort is way more reliable than any HDMI connection and less problems with the P3 color space in my setup. I’m down to two cables and only plug in the display, when needed.
All other connections (TimeMachine backup to external hard drives or plugging in my iPhone/iPad for a backup to my NAS) are done via USB-C-to-USB-A/USB-C/Lightning adapters/cables. They do not stay plugged in all the time, which is also highly recommended for hard drives that act as backup destinations.