Yes six months seems very long I recall when I switched to a new language (PL1/G) on the Map Reduce part of our project, the developer I took over from after 3 days said "you don't know it all yet"
I was teaching my self from a 1964 vintage IBM manual that my father had kept :-)
It's not just a new language(C#), it's also a new ORM (EF Core), a new front end (React or Angular or Razor or Blazor or whatever), a new database(SQL Server), a new cloud provider(Azure), a new web framework(ASP.Net), a new work tracker(Azure Devops)
It can be, but a senior who is well-versed in the fundamentals and has some exposure to complimentary tools in other ecosystems is not going to have a problem coming up to speed with those tools.
What they're going to spend more time on is: learning how you decided to solve your problems with your knowledge of those tools and how you decided to fit it all together.
If you have been around for a sufficient number of tech cycles you already have conceptually analogous systems in your head, which you just have to remap to the new syntax. That's what makes a "senior".
The frontends are actually the hardest to stick with because they turn over fastest, probably followed by distributed-compute solutions. C# is a short walk from Java, SQL has been around for decades(even given dialect differences that change best practice).
You aren't wrong that it is a bit of work. It's just less work than learning the actual software itself by a lot. Learning .NET's flavor of the year framework stack is not that bad since there is lots of good documentation, classes, books, and blogs. Your software, at best, has documentation that is as good as .NET's. This is highly unlikely though. Learning the product's layout and quirks dwarfs learning the latest incarnation of .NET.
I was teaching my self from a 1964 vintage IBM manual that my father had kept :-)