This is not just about the UI, it's about the mental model and management of the changes.
Just covering the review process:
Yes, you can structure your PR into 3 commits to be reviewed separately. I occasionally structure my PRs like this - it does help in some cases. But if those separate parts are large, you really want more structure around it than just a commit.
For example, let's say you have parts A, B and C, with B depending on A, and C depending on B.
1. I may want to open a PR for A while still working on B. Someone may review A soon, in which case I can merge immediately. Or perhaps it will only be reviewed after I finished C, in which case I'll use a stacked PR.
2. The PR(s) may need follow up changes after initial review. By using stacked PRs instead of just separate commits, I can add more commits to the individual PRs. That makes it clear what parts those commits are relevant to, and makes it easy to re-review the individual parts with updated changes. Separate commits don't give you that.
Stacked PRs is not a workflow I'd use often, but there are cases where it's a valuable tool.
Then apart from the review process, there are lots of advantages to keeping changes small. Typically, the larger a change, the longer it lives in a separate branch. That gives more time for merge conflicts to build up. That gives more time for underlying assumptions to change. That makes it more difficult to keep a mental map of all the changes that will be merged.
There are also advantages to deploying small changes at a time, that I won't go into here. But the parent's process of potentially merging and deploying the search index first makes a lot of sense. The extra overhead of managing the index while it's "unused" for a couple of days is not going to hurt you. It allows early testing of the index maintenance in production, seeing the performance overhead and other effects. If there's an issue, it's easy to revert without affecting users.
The overall point is that as features become large, the entire lifecycle becomes easier to manage if you can split it into smaller parts. Sometimes the smaller parts may be user-visible, sometimes not. For features developed in a day or two, there's no need to split it further. But if it will span multiple weeks, in a project with many other developers working on, then splitting into smaller changes helps a lot.
Stacked PRs is not some magical solution here, but it is one tool that helps manage this.