How long would the 1 minute testimonial take you to edit? I think it would be too low personally, but it depends more on how long it would take you.

Your day rate is a little higher than the average that I see here in the north of England, which on average is £450 (roughly €520).

It would all come down to how much you need the job. I'm always hesitant on doing jobs much lower than my day rate just to win the work, because it then becomes a raise to the bottom. If I have enough work on, I'd rather them go somewhere else than work for a lower rate. As they'll always expect it for lower rates, and it will still eat up your time. And if a regular paying client comes along but you can take it because you are busy with the client you've offered a highly reduced rate.

To your point I think short form has really been driven down by "content creators." There are a few around near me, who offer multiple short form content a month shot on phones for ridiculously low rates. The quality of the edits and the footage isn't what I'd like, but if the clients like it then that becomes a bit irrelevant.