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.

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.