By this same logic, the children of her British counterpart, Victoria, should also have been princes and princesses of Coburg, as well as dukes and duchess of Saxony. However, I don't believe any was styled such -- excepting, of course, her second son (the Duke of Edinburgh who succeeded his uncle as the reigning duke of Saxe-Coburg and Gotha).
All of Victoria’s male-line descendants had the titles of Prince(ss) of Saxe-Coburg & Gotha and Duke/Duchess in/of Saxony in their full
titles and styles. These titles were specifically mentioned in George V’s renunciation of the use of German titles and honours.
As for Bulgaria: the country was originally a principality, and only later became a kingdom; so the titles and styles of the future Tsar Ferdinand are extremely confusing. It seems that he started off as a Highness, then became a Serene Highness, then a Royal Highness, then finally Majesty.
Highness is above Serene Highness so he would mot have gone down and then back up.
My question is: just what qualifying rank did the house of Saxe-Coburg and Gotha originally belong to, anyway, regardless of the foreign thrones that members came to occupy? I read somewhere that in 1881, Prince August (second son of Prince Ferdinand and Countess Maria Antonia Koháry) and his descendants were bestowed with the qualification of Highness by Emperor Franz Joseph of Austria-Hungary. The reason was that his eldest son, Prince Philipp of Saxe-Coburg and Gotha, was married to Princess Louise of Belgium, whose sister Stephanie married in that year the emperor's only son and heir, Crown Prince Rudolf.
Ducal Serene Highness until 1844, at which point it was abandoned for the superior style of Highness.
The relative order of various forms of Highness in English is as follows in descending order:
Imperial (& Royal) Highness
Grand Ducal Highness
Ducal Serene Highness
Does this mean that members of the house of Saxe-Coburg and Gotha were originally only Serene Highnesses until 1881? It's hard to say, because Princess Sybilla (who descended from another line, not that of August) was born a Highness (in 1908). But then, it was probably because she was a dynastic male-line great-granddaughter of Queen Victoria of Great Britain: she was, in fact, born a princess of the United Kingdom. It's just that she got stripped of her British title later on, through the 1917 Letters Patent issued by King George V, who decreed that only children and grandchildren through sons of a monarch can style themselves as princes and princesses of the UK, with the qualification of Royal Highness. His Majesty stripped not only his Coburg cousins but also, his Hanover cousins of their British royal titles. In addition, he made his Teck and Battenberg cousins trade away their German morganaut titles for British peerages.
As above, Coburg was Highness in its own right after 1844.