Photovoltaic system 120 sqm living area - cover the entire roof?

exactly,
I get a different result:

Premises:
Roof pitch RP 35°, panels mounted flat. (Ecliptic 23°)

then, for example, at FFM (BG approx. 50°N) the south panels would see a max 52° angle of incidence in winter (at noon):
(90° - BG) + RP - Ecliptic.
For the north panels it would be
(90° - BG) - RP - Ecliptic = -18° max angle of incidence
meaning the north panels don’t see the sun at all from about Nov to Feb (when most electricity is needed).
They only get some diffuse radiation, typically < 5% nominal power.

That’s why north panels at 35° RP don’t make sense in Germany.

Or with a mounting frame?

ps.: Nominal power is defined at 90° angle of incidence,
at approx. 70° solar elevation and
1 kW radiation power/m²
(IIRC)

This cannot be calculated generally like that. It depends on which panels you want to use. If I am to believe the solar installers (and we have already talked to several), then the modern panels of the last 2-4 years have taken a huge step forward. You no longer need direct sunlight to generate an acceptable power output. Of course, you generate more with south/west orientation. But if you have the space and no shading, it also works easily on the north side. (we have a very large roof, so the north side gets enough light at 35°) With northeast orientation you naturally have to make more compromises.
For example, with our system it is not about always getting the maximum possible yield, but about covering our demand over the day for as long as possible.
In our case this means a battery for the heat pump and accordingly large system so that the adequately sized battery can also tend towards full capacity in winter.

To give you an idea, I have attached our house (this is from last May, according to the sun position it must have been around 10 am). The shadow of the trees in the west will be interesting in winter. But they will already calculate that accordingly.

You no longer need direct sunlight to generate an acceptable power yield. Of course, you get more with a south/west orientation. But if you have the space and no shading, it also works easily on the north side. (we have a very large roof, the north side gets enough light at 35°)

That would be nice, but I wouldn't bet on it. If you don't care financially, of course you can still do it - it won't hurt.

For our system, for example, it's not about always getting the maximum possible yield, but rather about supplying our demand over the course of the day as long as possible.

Well, in winter the sun only plays around in the south and is also low. And combined with my above-mentioned skepticism, the north won't bring anything in winter at first.

In our case, that means a battery for the heat pump and a correspondingly large system so that the adequately sized battery is aiming for full charge even in winter.

Note: it is important to have a sufficiently large battery inverter for that. Because if the sun comes through for two hours in winter, the photovoltaic output should be fed into the battery without restrictions as much as possible.

Are you sure? That would be bad, then nothing would be right.

I played around with the buttons in PVGIS again. Although you cannot set the panel tilt negatively (north orientation), you can rotate it around the vertical axis. That amounts to the same thing: Here are the values for 35° panel tilt with south and north orientation:

The results are roughly as expected: with north orientation, you only get diffuse light (approx. Nov - Feb). The yield degree is then only about 8% of the summer value, exactly when you need the most electricity. That is only about 1/4 relative to south orientation.

The diffuse light proportion is difficult to predict, it depends heavily on the panel construction and environmental influences.

Although you cannot set the panel tilt negatively (north orientation), you can rotate it around the vertical axis. That amounts to the same thing:

Therefore: PVGIS works well and is easy to use if you fill in the correct fields.

There is also a step-by-step guide for "ersuchmaschinen", even in German.