Looking for advice about the first job after graduating

Hello! I hope everybody is keeping safe and healthy. I'm graduating this December and accepted an offer from the company I was working part-time on while I was studying. For the country I am in, they gave a great offer to someone who is about to graduate. The company is a very small startup. The main reason I accepted it was I felt it was a good way to thank them, cuz they gave me an opportunity while I was still at Uni. Also, I read somewhere that it is a good way to grow on a codebase you worked on before. I wanna see the mistakes, the good decisions, essentially learn on the previous work I made while I was working part-time, well I hope I do see them :D

My question is should I have applied at other companies as well before accepting, because I have a good feeling I could've nabbed some offers on the well-known ones in our country (had commercial dev experience while studying, also a bit before studying. Had decent grades, had a non-trivial project from school - well for me it was).

There were a few companies that I felt were really good when they did that presentation thingys students attend.

Am I slowing my career growth? Because at the moment, this small startup is using older tech by web standards. Or is my previous reasoning better (grow on a codebase, give it some time, yada yada). I have a feeling I might not be able to go full on studying new tech to keep up outside of work, or is that a must anyways - essentially at most work places you will be working with old tech?. Seeking advice from experienced people, and thanks in advance for your time :)