Commits absolutely do NOT need to be some coherent chunks, they should be whatever state you feel you want to save. Work in feature branches, make as many or as few commits as you think is suitable, and then when merging use a good merge message describing the feature or changes.
Often if you name your branch feature/new_login and merge that, the common auto generated message that is something like Merged feature/new_login is descriptive enough.
If your workplace has some strict policies about commits having to be coherent, or planned, you should complain or work somewhere else. The policies and tools should be made so your life becomes easier, not harder, and if you need to plan when you can commit you're making your life harder.
What you SHOULD strive to do is keeping your clone in a state that you can always push and pull, and you don't need to be afraid of breaking something, or something breaking on your end.
I often commit when I've done a significant amount of progress or when I finish the feature or bug fix, and when switching tasks, not much more often than that. When I realize I've got multiple unrelated changes I'll commit them separately. I'm not scared of committing something that's broken or not complete, as I never work on a branch that's supposed to immediately go live, or someone else depends on.
Instead of planning your commits, you SHOULD pay attention to what you commit though. Don't do blind commits of the git add . && git commit -m "stuff" -style that way too many people do. Make sure you actually know what changes you are committing and that you didn't leave debug code or similar things around, unless you need it when you continue working.
git add . && git commit -m "stuff"
Use a GUI tool to diff the changes (your IDE should help here). Make sure you use a similar visual diff to re-check all your changes when merging your branch (or pull request). Beyond that there really isn't a need to "plan" things out.
Some of what you say makes it sound like your workplace doesn't do code reviews. Reviewing changes submitted as incoherent batches of commits is a pain, to put it nicely. Especially if they're large commits. Especially if they're so incoherent that there are a bunch of changes mixed in that are completely unrelated to the change I'm reviewing.
No, code reviews are really simple. Do a PR on GitHub, or BitBucket, or whatever you use and you get a diff of ALL the changes in a branch. If you don't use those tools, you can always do a merge locally and view the full set of changes. I've been doing this for quite a few years and there is no problem in viewing all the changes in a feature branch.
Just learn to use the tools you have instead of making everyone spend extra effort.
The tools can't exclude a bunch of irrelevant changes if they're submitted with the code to be reviewed. Instead of the submitter doing a little extra work they're forcing the reviewer to.
The tools can't explain why changes were made when a bunch of commits have a single comment that doesn't explain anything, like "feature completed". Instead of the submitter doing a little extra work they're forcing the reviewer to.
If everyone learned to use the tools well there wouldn't be these problems, obviously, but it's clearly not as black and white as "Commits absolutely do NOT need to be some coherent chunks".
So don't commit irrelevant changes to your feature branch, this falls upon the "blind commits" -category of problem.
The best way to explain why certain changes were made is not in the commit messages as you should not review separate commits but the end result of the work in the feature branch.
What I do if some change is unclear is usually one of the following:
Oh and if you don't know which of the changes you made are relevant to your feature branch anymore you don't commit often enough.
There's more people in this world, and in your company, than you, dude. Just because you feel like it's too much of a burden to write a good commit message does not mean the tool is getting in your way. The entire point of git is to have a history, to be able to look at it, and revert certain parts of it. Step one of doing all of that is having a coherent history.
I've been programming using DVCS systems for a living and in teams for quite some time, and know exactly what level of effort you need to put into your DVCS usage to make it easy for other members of the team.
I didn't say your commit messages should be "a" or "stuff", but there is literally no need to be super careful with your single commits, as long as you work on feature branches.
If you're not using feature branches (you should) you must at the very least make a very good commit message, but really you should be using feature branches. When you create your feature branches the name should be clear enough that you don't need to read the commit messages to understand what it's about. Any additional information reviewers might need can be attached into the PR, and the task/issue related to the changes.
If you directly make a single commit to master or similar for your whole feature, you're doing it wrong.