If I were interviewing you for an Embedded position I would ask questions about how your skills with .NET and React applied to your embedded projects. I would review your github repo and ask about the embedded projects there and what lessons you learned, and how they were different than a .NET project.

Changing specialties is rough, but I have done it more than once. Embedded is a strange space, but you can break in.

You could try transitioning more gradually by using your full-stack skills to create a website that you send data to via an embedded device (e.g. Pico or bare-metal system), either over Ethernet or WiFi. Even a simple dashboard with a table that shows e.g. HTTP or MQTT updates. For example, hook a temperature sensor up to a Pico or ESP32 (built-in WiFi/BT support), code it to periodically HTTP POST a JSON body with a timestamp, temperature value. Display the data on your website, maybe add tables and sorting, a graph(s) even, etc. That would make use of your existing skills, let you dabble in C and bare-metal programming, and combine the two to have something easily shown to prospective employers. (And show a product that is similar to existing real-world embedded applications)

I'm an embedded guy, the advice above is a reverse of what I've done in the past for clients: I wrote the embedded device firmware and dabbled a little in using .NET to create a portal for the customer to view and manage data from a fleet of devices reporting things like timestamp, battery level, temperature, etc.

I spent several years working on an embedded hardware project.

The key skill is C. Most everything else can be learned on the job, and will differ from project to project.

Obviously not all embedded programing is in C (although a lot is) but folks proficient in C are typically quick to pick up other languages.

So from a resume-bolstering point of view I'd suggest getting a Raspberry PI (or similar) and build some C projects on it. That would expose you to a lot of embedded skills. Bonus points if you choose a device either severe memory and/or cpu constraints.

I switched development careers from fullstack to enterprise API management about two years ago. I specialize in software focused data transmission from personal projects so I had no problems with the interview. If you can do enough personal projects to become an expert of embedded systems design and architecture enough to speak to edge cases and measurements from various performance scenarios I am sure you won’t have any problem transitioning.

In the beginning, make something that you find interesting and cool, and then gradually challenge yourself to more complex embedded products. I believe that you will become more and more powerful in the process of practice.