I also have interviews that don't go further, it's part of applying, especially when clients have 50+ applicants to choose from.
As for your question - my first client was in the B2B SaaS business - and I had no idea if I could deliver what he wanted. I had to edit (that was the easy part) and create outlines for lots of industries I had ZERO knowledge of. But with plenty of research, I made it happen, and the client was a repeat customer.
Generally, I apply to things I'm fairly confident I can achieve AND nowadays, it has to be something I enjoy as well - doing a job you hate can feel like forever.
But, every once in a while I shoot for the stars and aim for copywriting jobs that are a little out of my league.
I would suggest balancing your proposals - easy, medium and difficult, and apply at least 20 times per week. The risk of applying to jobs you're not very skilled in is getting bad reviews and ruining your score.
You can read my most recent article on Upwork to learn more. Hope this helps!