Cant set and save Custom white balance in memory

I'm pretty gobsmacked by this. I have the A7S3 and you can't set or even change what the custom WB was previously set to in any of the memory modes. So that means you can't even jump out to a non-memory mode, custom white balance and switch back to memory mode as it will still have the 3 custom white balances you set when you saved the custom memory mode in the first place, presumably at an entirely different time and place!
It's bizarre, we have this wonderful run and gun machine, which they've part crippled with this white balance nonsense. It's not even a new issue, Sony has seemingly made this a deliberate design choice for years. I mean someone must have programmed in the option to be able to choose which white balance mode you can set a memory mode to and then deliberately program out the custom white balance part of that! It must have taken more coding to exclude part of the white balance menu than to not have. Way to go Development Team!
You have to switch out of memory mode and hope you haven't overlooked some other camera setting which you'd previously carefully stored in a memory mode and shoot there if you want to be able to custom white balance.
The only other workaround I've found so far is to set a custom button to toggle Automatic White Balance Lock and then try and fill enough of your frame with a grey card in the light you want to measure and wait till the camera has settled and lock the white balance.
Why has Sony done this? My Nikon Z6 doesn't have this problem, I have a custom button assigned, which works in its memory mode and when pressed and held (if WB was already set to measure mode) takes and sets a custom white balance. Job done - unless you are mounted on a gimbal and have a USB C cable attached, in which case it just doesn't work...Why do they hate us?