@Nathalie-Kranich Keep going. your work really comes a long way. Regarding your last question, I would say do both, pick out a few images you want to improve, and work on them, keep making new stuff also. But at the same time start querying more.
I am in a similar situation as you. Right now I am working on improving 2 pieces in my portfolio, and making a couple more spot illustrations. I am also researching which agencies to contact. I want to send out to 3-5 of them and wait for 2-3 weeks before sending out the next batch. I just assume most of them will say nothing, and maybe one will say "no"( which they really mean is "not yet"). If I do get a "No", then I can try to keep the conversation going by asking "what is missing from my portfolio?".
I used to think to query agency is like applying for a job. But now I think of it in the opposite way. It is more like I am offering someone a job (it is just chances that he/she would reject the job offer is very high). It helps me to think this way because I realized that I have to make sure to offer the right person the job of representing me by doing my own research.
Hope this helps.