Distinguish opinion from suggestion while reviewing a manuscript?
You have space. If you have a scheme of what to do, just follow it. Read further if you don't have any scheme:

Your purpose as a reviewer is typically NOT to make recommendations on how the manuscript can be improved. You are NOT asked for suggestions how to improve. You are asked for an evaluation. I suggest the following scheme:

[

Summary:

...

High-level issues:

X is not a valid conclusion because of y. It is wrong to include x into the paper.

...

Low-level issues:

...

Improvement suggestions:

Perhaps it would be beneficial for Z to be added. Consider including Z due to W.

]

Include "improvement suggestions" only if you really wish to or if you are asked for that explicitly. If in addition you are asked for "nondisclosed comments to the editor / program-committee" or something else, then include such parts, too.