I honestly just winged it. I came from C++ so you're in a much better position to learn .NET than I was. There's little need to get into the syntax. You can pretty much just wing it. It's just OOP after all. You could probably look for some crash courses so that you can speedrun the language differences but other than that, you'll do just fine using the official documentation.

Microsoft has free resources for their frameworks.

https://dotnet.microsoft.com/en-us/learn

I can’t speak to the specifics of transferring from Java to C#, but as someone who recently had to re-learn/catch up on it and Dotnet for a job transition I can say what helped me after not touching it for nearly a decade.

I found a few people on YouTube that did project walkthroughs, and I followed along with them and built out their examples. I used C# 10 In A Nutshell as a reference when I felt I needed to dig deep on the various things the videos touched on or that I knew I’d be needing for the code base I was inheriting. I was preferential to Tim Corey, mostly because a lot of his videos felt up-to-date with the latest technologies and were easy to follow. He also has courses designed to be a boot camp for getting up to speed, I believe, on his website if that is an option for you.

Not an answer, but I've never had cause to learn anything about .NET (F# looks good though, i guess) and I hope that continues

It’s very similar. I’d say learn syntax differences and read/watch some basic tutorials and the rest you’ll learn along the way.

You seem to already know what you are doing and have experience with Spring Boot. I'm not sure if you're looking into also developing web applications for .NET, but have a look at the official ASP.NET Core documentation. There's a whole lot to be found there, and examples on how to develop different types of applications. Go through one or more you are already familiar with, and you'll find the .NET ecosystem and the C# language really isn't that much different from Java. The language itself shares most of its paradigms with Java, and even syntactically it is very similar.