similar to the Highlands in Scotland or Scandinavian fjell
Not in Germany.
Although they share some similarities at a first glance, Scotland and Scandinavia have become two quite different landscapes due to a different history of human impact.
In Scotland (and other hilly areas in the British-Irish isles), well over 99% of the original forest was removed by humans (historically the British Empire needed wood and today recovery is inhibited by a powerful sheep-and-nostalgia lobby, not only in Scotland but also in England and Wales, to the degree that the barren Lake District was declared a — cultural, not natural! — World Heritage Site), and extremely little of the Caledonian Forest remains. German (and more broadly Central European) culture has historically and presently have a much higher valuation of ecology. Many areas that were cleared have been reforested. I don't think any of the German Mittelgebirge have large deforested areas of land left (with droughts, storms, and plagues, there appears to be a large risk that clear-cut areas will return in coming years, though, but the ground is different and it will not look like the uplands of the British-Irish isles).
The Scandinavian mountains is very different because much it is naturally above the tree line (almost none of Scotland is above the tree line), and many areas below the tree line have either restored or recovered their mixed forests due to a (much) lower density of sheep (compared to Scotland). In Germany, you will only find tundra landscapes in the higher regions of the Bavarian Alps, but it's not like Scandinavia. The mountains are steeper and busier, the valleys are deeper and narrower, opportunities for hiking off-trail are limited or absent, and you can only sleep in official accommodation, which may not be open this summer (classical mountain hut accommodation may involve many people sleeping side by side in a single huge bed, not a good situation for slowing down the spread of a contagious disease). The vegetation in the Alps is also quite different from Scandinavia. Small pockets with moors or berries do exist in the Alps, so you may find spots that remind of Scandinavia, but I don't know the Bavarian Alps well enough to recommend any in particular.
If you can tolerate forests, there are many opportunities in German Mittelgebirge such as Erzgebirge, Spessart, or Schwarzwald. They are much more densely populated than the Scandinavian mountains, and if all Germans can vacation only domestically they may get even busier than usual, as they're not famous enough to draw a large crowd of international tourists as far as I'm aware. Fortunately, the density of trails is very high so it's quite possible to hike all day without meeting people (it helps if you go mid-week, off-season, in poor weather) but you will still be close to civilisation.
(If someone is in the reverse situation, located in Great Britain and seeking naturally forested hills: the only ones I've found during my time there was northern Exmoor, which I found very beautiful and reminded be of hills in Germany.)