Topsoil or potting soil better for new planting?

Whether or not to mow depends on the concept. The wild carrot is an all-rounder in this regard. It grows in the flower meadow (2-3 cuts per year), but also in edges ([eimalige Mahd im Frühjahr]).

Do you know of a plant that has the properties/advantages of wild carrot but blooms yellow? By the way, I read in my app that wild carrot should not be mowed because it will look really good in winter as well. I'm curious.

So, there are several plants that bloom yellow, look great in winter, and offer a lot for insects.
I love true bedstraw, Galium verum, but it prefers rather nutrient-poor soils (an important caterpillar plant), and it also smells really wonderful like honey. For the effect, though, you need more plants.
Definitely for normal soil, the yellow scabious (Scabiosa ochroleuca), an insect magnet and beautifully sways in the wind :)
Common toadflax (Linaria vulgaris) reminds me of snapdragons and is really pretty, in my opinion!
The large salsify (Tragopogon dubius) makes up to 76 wild bee species happy but only blooms until midday. However, it has really great huge dandelion clocks.
Spring cinquefoil (Potentilla neumanniana / verna) is important for 17 caterpillar species and makes many bees happy.
Coltsfoot (Tussilago farfara) is an important early bloomer that blooms from February and only produces leaves afterward. It likes it dry and doesn’t need watering. I also find the leaves really nice (which is important because it blooms early until April and afterwards only "shows leaves").

All native hawkweeds (Hieracium pilosella / piloselloides / lactucella etc.) are rather inconspicuous but absolute winners for insects.

For later in the year, a gold-hair aster (Aster linosyris) might be something.

The great mullein (Verbascum densiflorum) and black mullein (Verbascum nigrum) like it a bit leaner. Verbascum phlomoides and lychnitis also like richer soil. They grow nicely tall and after flowering their flower stalks serve as insect skyscrapers :).
If you like herbs, dill blooms very cutely, is not native but edible for that reason :)

In general, it’s good to ensure that something blooms from February to September.

I read that you don’t really like pink, purple and blue, but I love the combo with yellow :)
and for our insects, these are by no means unusual colors, e.g., bugloss (Anchusa officinalis), viper’s bugloss (Echium vulgare), knapweeds (Centaurea scabiosa, montana, jacea, nigra) and field scabious (Knautia arvensis) are pink, purple and blue and extremely important native wild plants.

For shrubs, depending on space, I would recommend cornelian cherry (blooms beautifully yellow, very early in the year), hawthorn, alder buckthorn, goat willow, and a serviceberry (very important, not the copper serviceberry, but the native Amelanchier ovalis) :)

I hope you find something beautiful!

Our garden is designed according to the Hortus principle, so it's a three-zone garden, there are great videos and info about it. Maybe that will give you some inspiration. I can already see your Hortus "Flavum" :) Almost all gardens have Latin names, ours is called Hortus Chiroptera, because of the bats :D

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Wow, what a treasure trove and what knowledge you have. I have already printed out your suggestions and will implement them step by step. Such great recommendations, thank you for that.

Oh, I’m blushing. It’s my "special interest" and I’m really passionate about it. I just spent another 2 hours digging in the garden :)

You might only get some plants through seeds, and the hawkweeds are considered "weeds" by many, but nowadays you can even buy them quite well online. I have many hawkweeds growing that have self-seeded :). There are always quite a few beetles and especially very small wild bees to be found there.

Oh! Not native, but absolutely great and important for moths, the scented evening primrose (Oenothera odorata).
Smells heavenly to me and the moths love it (and the bats love the moths ;)).