Best practices form processing with Express

I'm writing a website which implements a usermanagement system and I wonder what best practices regarding form processing I have to consider.

Especially performance, security, SEO and user experience are important to me. When I was working on it I came across a couple questions and I didn't find an complete node/express code snippet where I could figure out all of my below questions.

Use case: Someone is going to update the birthday of his profile. Right now I am doing a POST request to the same URL to process the form on that page and the POST request will respond with a 302 redirect to the same URL.

General questions about form processing:

1. Should I do a POST request + 302 redirect for form processing or rather something else like an AJAX request?
2. How should I handle invalid FORM requests (for example invalid login, or email address is already in use during signup)?

Express specific questions about form processing:

3. I assume before inserting anything into my DB I need to sanitize and validate all form fields on the server side. How would you do that?

4. I read some things about CSRF but I have never implemented a CSRF protection. I'd be happy to see that in the code snippet too

5. Do I need to take care of any other possible vulnerabilities when processing forms with Express?

Example HTML/Pug:

form#profile(method='POST', action='/settings/profile')
    input#profile-real-name.validate(type='text', name='profileRealName', value=profile.name)
    label(for='profile-real-name') Name

    textarea#profile-bio.materialize-textarea(placeholder='Tell a little about yourself', name='profileBio')
        | #{profile.bio}
    label(for='profile-bio') About

    input#profile-url.validate(type='url', name='profileUrl', value=profile.bio)
    label(for='profile-url') URL

    input#profile-location.validate(type='text', name='profileLocation', value=profile.location)
    label(for='profile-location') Location

    .form-action-buttons.right-align
        a.btn.grey(href='' onclick='resetForm()') Reset
        button.btn.waves-effect.waves-light(type='submit')

Example Route Handlers:

router.get('/settings/profile', isLoggedIn, profile)
router.post('/settings/profile', isLoggedIn, updateProfile)

function profile(req, res) {
    res.render('user/profile', { title: 'Profile', profile: req.user.profile })
}

function updateProfile(req, res) {
    var userId = req.user._id
    var form = req.body
    var profile = {
        name: form.profileRealName,
        bio: form.profileBio,
        url: form.profileUrl,
        location: form.profileLocation
    }

    // Insert into DB
}

Note: A complete code snippet which takes care of all form processing best practices adapted to the given example is highly appreciated. I'm fine with using any publicly available express middleware.

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