I have decided to revert to OpenWRT v19.07.07, the current stable version.
Lantiq has been an Intel subsidiary for a long time now, and the Home Hub 5A is a very old piece of hardware. I doubt there is ever going to be any official "fix" to the time drift problem.
Until some way of fixing the drift can be found in the 20.02 series, it doesn't matter whether it is something built into the release, or some mod that a user implements, as Handyman has done for 19.07, it's effectively broken for this hardware.
It's a pity, it's good hardware, even after all these years.