Electric shower is cold in the morning
How cold? (20°C, 30°C)
Remember that in winter mains water is colder (5°C vs summer's 15°C) and at night it gets even colder: 2° On average, and with a 8,5 kW you can rise only 3,5L/min at 35°C delta (which is really low flow).  It means a 37°C shower in morning and 40°C in evening.

I think its a too low power appliance: consider switching to instant-gas that allows by-far better performances. Even going for a 10 or 12 kw electric shower won't improve much your situation. Remember: a flow of 1L/Min (starting at 5°C) requires a power of 2,4 kW. And a good shower (at least in my Italian terms) is 10L/min, although 6L/min is still acceptable.

Another electric option is to install a Point-Of-Use immersion that can give you really better performances (for 2-people shower 40L is enough), at least in winter.