I'm working on a project in which the number of contributors is small, 2-3 engineers.
I know I can suggest code changes in an existing pull request, but I'm working in a capacity where I've been asked to advise, not code. So it seems a little weird to create a fork/branch of main/master, and create a pull request against the repo in which nothing has changed, and suggest changes via commented code (which would list out the changed files where I want to make suggestions).
I want to suggest changes where I can just highlight the code and a comment box pops up, add my notes, like how it works in a normal pull request.
I know of gists but I'm not quite sure that is really what I'm looking for.
Note - I realize what I'm asking for is probably ridiculous because anyone with a github account could suggest a change to a popular repo and leave a comment "THIS SUCKS!" but yeah, this is assuming that the suggestions are limited to accounts associated with the project.
Open a ticket in the team's issue tracker. Be as specific as possible about your suggestion.
If the team isn't using an issue tracker, then they really should start. GitHub has a decent built-in one (called Issues), or you can use an external tracker (Jira is the best-known but there are others), but the important part is keeping track of work that needs to be done (bugs, suggestions, new features, etc.).