I’ve also been watching this too for a while and given the fact that weekend prices are usually a lot lower it’s ok for me to have it active on weekends only because it usually will store as much cheap energy as possible until Monday leading to better overall efficiency for me.
And (see e.g. DESS next-day schedule based on simple price prediction) I would keep this very simple, with lookup tables being my favorite. E.g. the cheapest reasonable forecast for the next Monday would be the prices from last Monday. Can be made more fancy (with seasonal averaging, etc), sure, but I think it’s not worth the effort and there will always be complaints.
And for the matter of SoC reserve - I’d like to have 2 reserves: a min SoC reserve for cases of higher than expected consumption during high price periods (that’s something I have already covered with a homeassistant automation) - as well as a max SoC reserve for cases where there is more sun and/or less consumption to prevent unnecessary grid feed-in. But this greatly depends on the current season and specific installation parameters (battery size vs. PV peak power vs. daily average consumption) and for the generic user this would make DESS too complex, so this should be hidden behind kind of “advanced settings, don’t mess with them and then complain, you have been warned”.