I more or less had a similar first experience, joined a startup with mostly no prior working knowledge and after a very short period the company split and I was left alone with the founder, who was a business guy, as the only developer.
had to learn stuff in the hard way, and probably a lot of I did was wrong, but I ended up setting up the website myself (db to frontend, back then in GWT, because had no idea about js and no time to learn), handled a mailing server and an ldap one all while working and being paid for a part-time job.
it was very useful and opened up my mind to way more than software development, having an idea of everything that goes into a real full stack really helped me later on (just as an example, though not the resident expert, in a later company I was the one introducing git and having the first presentation and demos on docker, and personally I point back at that first experience).
what was maybe different than your situation is that, for whatever reason, the founder really trusted me and I had completely freedom on the tech to use
I guess, what I want to say, make lemonade out of lemon, find your way and I think a real experience is way more formative then any pet project, as there are real consequences for failing