I believe there is no upper limit possible to be set to stop charge at 80%.
If you enable Time of use (TOU) then it can stop at 80% for grid charge at the desired power only. It will still charge towards 100% if you have more solar than load.
What you can do is to change charging power in TOU to 0 somehow on-the-fly but not too often.
Note that it is very difficult to figure charge state for LiFePo4 batteries due to flat charge profile.
The only way BMS to know it is full is by charging to 100% and when the charging current drops BMS knows "it's arrived". Tesla for their LiFePo4 powered cars recommend charging to full at least once a week and not lowering charge limit below 100% to be sure that it gets full hit for regularly. If you are more clever then you could charge to less (80%) and prolong battery life for few years. Note that 100% is not that harmful for LiFePo4 batteries if you do not stay at full charge for longer periods. So please discharge your batteries soon for your home usage to some 70% and use peak shaving if you need it to go down to battery low level (20%). This way you will calibrate the batteries once a day or less if there is no sun.
I believe it is difficult for implement something more clever than what Deye offers now. I would also like to have more control on how and what percentage would the batteries be. The only way would be to put some controller (Raspberry Pi) between batteries BMS and CAN/485 port and report fake percentage to Deye to change/discharge at our desire such as sun peak shaving, etc.
