Reviewers on separate forms, comments can:
go to ... > Set Status > then select Accepted, Cancelled, Completed, Rejected.
Reviewers in share mode are even more limited:
... > Resolve
These are inconvenient paths to say you agree with something, also all these words imply finality rather then collaborative thinking.
Option 2: Reply by writing out the words.
When going over a 100+ page document with as many comments writing a full reply is time consuming.

Adding a thumbs up emoji/icon or even give a stamp option inside of the reply that says agree (regardless of the review type) would be much more convenient and collaborative.