The GARDENA gardening expert
Dear Ms. M., generally the arbor vitae variety you have mentioned (known as the emerald green arbor vitae) is itself an excellent plant for easy-care hedges, as it hardly needs cutting. However, it can indeed reach quite a height.
If you are looking for something which grows more slowly, then I would suggest for your sunny location the Irish juniper (Juniperus communis ‘Hibernica’). In general, there are a number of upright column-shaped conifers which can cope with sun, and which remain fairly small. These “mini-varieties” do, however, as the description suggests, often require many years before they reach even a half-way acceptable hedge size. This makes these “adult mini plants” relatively expensive, as you literally require a whole row of them. With the ‘Hibernica’ variety, however, you are well-supplied with a reasonable price-performance ratio.
As far as your request for the solution with shrubs is concerned; here your choice is subject to various restrictions, as you require the types and varieties of plant which can reach hedge size. Due to the way they grow, shrubs aren't really meant to act as a barricade.
In order for the people sitting behind it to remain unseen from the outside, you would require a growth height of approx. 150 cm. The following plants can achieve this height: larkspur (the Elatior and Pacific hybrids), Helianthus microcephalus and H. salicifolius (sunflowers), Macleya cordata (plume poppy, up to 3 metres), giant miscanthus, giant cane (in the variety ‘Variegata’, also in the colours white and green), tall moor grass (Molinia arundinacea), or dark mullein (Verbascum nigrum) – depending on their location and your design requirements in detail. Simply look around your garden centre to find your favourite variation.