What happened?

Sadly Google is shutting down the free version of G Suite for good.

What now?

There are some not-so-good alternatives:

• Self hosting: Mailcow is extremely memory hungry and delivery is definitely spotty unless you use a external SMTP provider like Mailgun.
• Yandex: Free, 10GB space, 1000 seats. Anti SPAM is very hungry and sending can be limited for no good reason.
• VK Mail(biz.mail.ru): Free, unlimited space, 5000 seats. Delivery is spotty.
• MXRoute: Good service, reliable. Price can be as low as $10/yr/10GB during black friday. 1GB mailbox can be as low as $3/year, 2GB at $5/year. There's a $179/lifetime/10GB package. Check Nexusbytes - price is better with resellers.
• MS Exchange: $4/seat.
• Office 365: $99/yr/6 seats.
• Paying ransom to Google: $72/year.
• Apple: $1/seat/month. No unlimited alias.
• Zoho: Free, 10GB space but NO SMTP/IMAP support.
• Disroot: Paid at 0.15 Euro/GB/yr. Delivery can be problematic.
• Protonmail: $5/sear/month. NO SMTP support.

Solution

So what I am looking should have:

• Large enough space(I've accumulated ~8GB of Emails in the last 15ish years)
• Good delivery
• SMTP support
• Unlimited aliases
• Unlimited seats
• Catch-all address

Gmail has a unique feature(not that unique but I've only seen it on Outlook outside Gmail) - "send as alias". You can setup a preferred alias, customize SMTP credentials and Gmail shall call that SMTP server when sending outbound Email when using that alias.
So what we can do is:

• Get Email forwarding service on the domain to catch Emails and forward to your personal Gmail
• Get SMTP working
• Add SMTP as alias in Gmail

Preparations

You will need

• A Email forwarder(will elaborate)
• Preferred SMTP provider
• Gmail account(can be free ones)
• DNS access of your domain

Email forwarder

A Email forwarder should act as your MX record, take inbounding Email and forward to designated address.
There are quite a number of good free forwarders around:

• improvmx
• Cloudflare Email Routing. Note you need full zone(aka using Cloudflare as NS)
• Mailgun: no free plan though
• Your domain register probably provides this feature for free
• Self hosting
• Sparkpost with Heroku

SMTP provider

There's no lack of them with free plans:

• Mailchimp
• Sendgrid
• Sparkpost
• Amazon SES(note traffic is not free)

Steps

Make sure you stick with this sequence.

(Optional) Migrate existing Emails

You can do that in Settings.

Setup Email forwarding

Go to any forwarder and setup MX and SPF record with DNS provider: MX record to receive Email, SPF to send Email on behalf of your domain so you can receive forwarded Email.

Setup SMTP

Use any SMTP provider and verify your domain. Add necessary DMARC and SPF record: the SPF record may looks like v=spf1 include:spf.improvmx.com include:_spf.somethingelse.com ~all
Note down the SMTP credentials. Using Sparkpost as example -

Host smtp.sparkpostmail.com
Port 587
Alternative Port 2525
Authentication AUTH LOGIN
Encryption STARTTLS
Username SMTP_Injection
Password an API key you created

Setup Gmail

1. Go to Gmail's settings and select "send as alias". Put in your desired alias.

2. Click next: Gmail may infer some IMAP settings - replace with SMTP credentials.

3. Click next: Gmail will send an Email to that alias for verification - you should receive that Email in your Gmail as you've already setup forwarding. Put in the code.

4. (Optional) Set the new alias as default.

Now you've got a reliable free Domain Email for completely free.