GitHub features the same draft system you described.
I thought theirs was immediate, but I was referring mostly to Bitbucket Server's system from previous experience with that.
GitHub has both. You can add an immediate comment, or add it to a draft that you post as one unit.
This is one reason I liked Phabricator’s review system, which allowed drafting comments on an entire PR before submitting the comments. This allows you to be as nit-picky as you want when reading the PR, and then delete or modify any of them at the end. Instant submission of line-level comments, on the other hand...
I think reviewers should have the awareness to understand not to over-burden the person whose code they are reviewing.
For example, if I’m reading some very junior code and finding lots of issues, maybe I should delete all my comments about minor things like style and only leave the comments requesting major fixes like code design or potential bugs? When the developer re-submits the code after fixing the main issues, maybe it will be less frustrating to get a review back with “this looks great, can you just fix these small issues that don’t conform to our style guide and it’s good to go.”?