What counts as a substantial revision?Generally if an agent says they'll take another look, they've also said something else. Hopefully they've given you a sense of what needs to be changed: stakes, pacing, tension, the ending unfolds too quickly, your characters are all named Gertrude.
I have recently made changes to a work I'm querying that included working with an editor who writes in my genre, cutting about 11K words, increasing the pace, clarifying stakes, etc. so a pretty big rewrite, but it's still fundamentally the same story, albeit shinier. I received a few rejections on this book that specified that if I were to revise the agent would be willing to look at it again. How much needs to change before an agent considers it revised? Is the best approach just to reach out detailing the changes and ask if the agents are still interested?
If you've fixed those things, it's a substantial revision.
If you haven't it's not.
The question then becomes how do you know if you've fixed them. What you did is exactly the right way to find out: get another set of eyeballs on your manuscript. It doesn't have to be a paid editor but it should be someone who knows a thing or two about good storytelling and knows if you've fixed the stakes, pacing, tension and denouement. You're on your own with the name thing of course.
But the real trick on revisions is not doing just what the agent suggested. It's taking what the agent suggested and thinking about what ELSE needs to be revised if those things are off.
Nothing drives me crazier than having someone say "Hey, I fixed those five things you talked about" and clearly not having thought that those five things also lead to six, seven, twenty four and ten squared minus two.
Revising is where the real writing comes out.
It gets harder to revise well the more familiar you are with your work.
I can tell you one thing for sure: substantial revision takes a while. Like months.
So if you've revised then gotten fresh eyes, then let the ms lie fallow for a time, THEN gone back and really looked at what you've got, then you just might be substantially ready to resubmit.