Welcome!
I expect someone will be along to suggest Home Assistant and outline how to set it up. From my question for a slightly different requirement at RPi Home Automation Software HA seems to have several experienced fans on here.
I’m currently using domoticz which is smaller and simpler, and has timers attached to things as standard, but has fewer plugins available, so it depends how you can talk to your GSHP and battery controls, which I don’t think you’ve mentioned? (or maybe I am too dim to realise that it’s obvious from a “Click PLC with an arduino” and “growatt off grid inverters”).
If they support MQTT with Autodiscovery, it should be a fairly easy setup in several controller software. The timer interface is very much a list of on/off instructions, not a chart to draw bars on, though. I find it easy enough to set up some timers quickly, but I’m not sure I’d want to do it every day. My ASHP control, which varies every day, is decided by an event script, reacting to temperature sensor events. It doesn’t react to Agile energy prices yet because I’m not on that tariff, as I think the way the Energy Price Guarantee has been implemented makes Economy 7 slightly cheaper for our needs.
I’m sure I saw a chart interface timer while looking for a control I liked. Maybe I’m thinking of GitHub - ioBroker/ioBroker.scheduler: Create and manage schedule profiles. for ioBroker (I deleted my ioBroker test for other reasons, so I can’t check in practice), or maybe it was one of the many add-ons for Home Assistant… if Home Assistant, I hope someone will mention it.