Is a venetian blind necessary on the east side?

I would decide here based on budget, room usage, and exterior appearance, less so on heat protection.

We have a 12m2 corner room on the ground floor as an office with south/east-facing windows, both with venetian blinds, because we didn’t want curtains and also didn’t want a mix of roller blinds and venetian blinds in one room. We have a few trees in the east and the sun is low in the morning.

I noticed that this summer until just now I forgot to include this exact east window in the automatic timer for heat protection. It makes no significant difference whether the window is shaded or not. Also, our front door with a lot of window area faces east; the hallway behind it hardly heats up. The house is solid with a lot of thermal mass inside.

On the upper floor we have roller blinds on the east for the bathroom and bedroom, and I would not want to do without them.

External shading always helps, but if the windows are not huge, you can also do without it in the east.

Like others, we mixed. We only have [Raffstores] in the main living area, that is the living, dining, and kitchen areas. The rest are roller shutters. We would do it the same way again. This way you have shade and light in the main living area, where you spend a lot of time. And you have darkness in the bedrooms. Everyone has to decide for themselves how they want it.

We also have venetian blinds in the all-room (south+west) and in the office (south). All other windows have electric roller shutters.

Or "isn't that done" and one should install blinds everywhere if one already does it in the south and west?

I often have the impression here that people do whatever is "hip" - without regard for sense and reason. My reason says: first) East is the morning sun side, and morning sun typically doesn't shine in brutally; second) enjoyment of summer is considerably spoiled if one cannot enjoy the view of the then blooming landscape because, just to prevent the living room from becoming a sauna, one lives all season long during the day in "low-flyer darkening" mode. So I would primarily operate with reasonably sized windows and assign each window its optimal shading.

Ultimately, it would have to demonstrate the thermal insulation verification for summer heat protection, here possibly according to DIN 4108-2. Often mixing is done for cost reasons, but I prefer a consistent architectural language here.

I would therefore primarily operate with reasonably sized windows and assign each window its optimal shading.

I agree, but the optimum is often not affordable. For south-facing windows, for example, a suitable roof overhang that sufficiently shades these windows would be the best. However, this often gets really expensive (e.g., with a concrete flat roof with ISO baskets, etc.)...