Difference between Gmail and G suite and even Zoho for mail sending purposes

So imagine you want to receive mail to Gmail from a custom domain mail of yours, and send STMP with a transactional maill hoster like Mailgun.

So from what I know, there are two ways of doing this.

If your registar is having the option to forward mail (looking at Cloudflare), you forward it to Gmail, you create an alias in Gmail with your custom domain, you choose STMP of google's or any other free or payed STMP services, but for this example let's use transactional mail providers like Mailgun, Sendgrid etc. You receive a confirmation mail to your custom domain, it forwards it to your Gmail, you got it working.

The second option, you choose not to forward it and maybe use Mailgun (you don't forward it, but you set up mx records within them, just like in G suite) and then you choose Mailgun forward and do the same again on Gmail site. This will probably be a better option, as it might do a better job than your registars mail forwarder.

But why G suite, why Zoho, why anything else than this simple and free way of doing? Even if you want to send 100k Mails a day, well you don't have to worry about G suite or Zoho, you will need to worry what STMP server you should choose, receiving mails are just fine when having the MX records at your transactional mail provider, so why would anyone buy Zoho, which is limited to 30 alias, a also G suite which I also think was limited to 30, but it might've been sixty, when I couldn't find any limit on Gmail free option, or am I missing something?

TLDR: Gmail doesn't let you set up custom MX records, but it allows for what I see infinite number of alias and setting up your STMP. Forwarding mail from registar exists, but also from transactional mails (arguably preferred). Mailgun has 10k transactional mails, great reputation, free. Transactional mail providers will let you set up the MX records there so you forward the mail from there to your free Gmail, you set up your Gmail to use Mailgun's STMP with no limit on alias that you can choose from and you send from Gmail for free. Why limit yourself with Zoho's 30 alias, G suites (I also think 30) at a cost larger or smaller, when you can do it all for free without no quality of service loss of mail sending, not privacy, but simple mailing.