When I'm doing developmental edits, I often run into the same issues across manuscripts and authors and even genres, so I wanted to offer a (not at all exhaustive) checklist you can use to help determine if you're ready to submit to an agent or editor yet.
Of course, please be sure to read, understand and follow the submission guidelines listed on the agent's or editor's or publishing house's website - the #1 way to get rejected is to not follow instructions. Below are a few more suggestions to help you avoid that "thanks but no" response.
If you would like some great resources on writing craft, check out Debra Dixon's GMC and Gwen Hayes's Romancing the Beat (specifically for romance authors, obvs). Both books are easy to read, straight to the point, and offer lots of great examples to help you understand the concepts.
Those are affiliate links to Amazon. That means if you click them and buy something from Amazon, I may earn a small commission at no additional cost to you.