PV system for the whole roof or better to undersize?

That may theoretically be true, but you also don’t calculate that you are missing out on x€ monthly over 20 years without photovoltaics due to your higher grid consumption, which you could, for example, invest in ETFs with a 7% return.

That is exactly what you consider in a proper calculation.

For me, it’s not about pros and cons or squeezing the last penny out. I enjoy my photovoltaics, as I do the e-mobility.

That’s exactly what I’m saying too. It’s purely a fun decision. I don’t enjoy it. If the original poster enjoys it: fill the roof. If not, then don’t.

I would definitely size the inverter as a hybrid and for 10kWp right away. Eventually the storage and the additional 4 kWp will come, then you buy a new one. I am in favor of going all in from the start.

As of today, I would stick to the old 10 kWp rule of thumb and not fully cover the roof anymore. In the Netherlands, since last summer, you can clearly see how the spot electricity price develops when a significant amount of solar power is added. At times when you have full yield, everyone else also has their peak performance – ergo very low spot prices on the electricity price exchange.

One can benefit from this with variable electricity tariffs if they have large controllable consumers. From my OW system, a maximum of just over 5 kW comes out; for the remaining kW, I would have myself paid in the future when charging the electric car. Just a thought...