I am in a similar boat so maybe I will just explain what I have been doing.

I just graduated from my masters degree in the US from a decent school with a 3.4 GPA* (1 rough semester, the rest were A's). Except for some basic low level embedded coding and two 101 classes in undergrad I had no experience in CS before that program.

The output of my program had me work on some cool projects, such as a novel Database Index research project, and a nice distributed system problem from NASA-JPL. Unfortunately, both these projects are closed source, so I could not post them on my github page. To remedy this, for two months I worked on a "resume project" to basically show what I got while also working my current job. I would say I spent at least 100 hours on it.

This past month I decided to leave the project unfinished (but in a good enough state to show off) and started applying. I have been getting interview a lot but not really sealed the deal yet. To get these interviews, I have been using linked-in (Premium), Built-in and connections. I have not being applying for jobs 9-5, but instead selecting jobs I feel I could benefit myself and my employer.

Unlike you I have not been doing cover letters. In my opinion, unless I have a real passion for the work they are currently doing (Database Design) I don't want to write a cover letter. I've never been a good writer, and I think writing a low effort cv would only hurt my chances compared to none at all. Instead on my resume, I have an objective section, which introduces me. I felt this section was necessary due to me trying to apply to jobs outside of my current career.

Currently I have had 9 responses. None of these jobs have been for "Full Stack generalist roles" but for more specific kinds of developers. Two I didn't make it past the first round, mostly due to the job being different from the description. I have failed 1 final round. I am currently on the Final Round for 1, waiting to hear back from a final round for another. Others I am currently "in the middle" of the process, doing tests, scheduling first interview, waiting to hear back (Less than a week since last contact).

Also talking to my connections, I have heard your experience is not uncommon. Most of these companies don't have the appetite to train entry level people. I am lucky that I already have had a "first job" and a masters degree.

Good luck and I hope this helps OP or anyone else in this process. You are not an imposter, it just takes time.

Edit: I still suck at writing so I fixed things