Sewer channel at the property boundary

That was more or less clear to me, but still, there cannot be a general prohibition on wastewater pipes at the property boundary (unlike with above-ground buildings). It was already mentioned that one should be careful when digging there.

You can't build too close to the border, otherwise the fence will fall in. But it sounds like GFL (right of way for walking, driving, and utilities) on the 3.5m, 40m long. Besides sewage, there also has to be water, gas, electricity, cables (TV or fiber optic network), video + doorbell and buzzer, gate drive 220V, path lighting, and during the construction phase it can also be a construction road for 40t.

Hello Nida35a, it only has to be video + doorbell and buzzer, pathway lighting through there. I get all other utilities from elsewhere. It won't be a construction street either.

You can't build too close to the border, then the fence will fall in.

Somehow I have to prepare the path to the neighbor's fence so that I can later pave a footpath...

For a path, you might dig about 40 cm deep, the foundations should be deeper, so no big problem for the short time.

A channel is rather 1 m deep and therefore more problematic.

Take a comfortable width for the channel so that a mini excavator can be used. So about 1 m away from the fence.
Then the distances from the shafts to the fence will also fit. Maybe don't plan the shafts directly next to a fence post.

If there are only inspection shafts for you privately, a 45-degree T-piece and a vertical pipe section upwards with a sealing cap are sufficient. It is important to have a slope of 1cm per 1m length and to rarely (never) use the shortcut key.