What should I use for “recovery email” for main personal Gmail account?

Do not chain to another google account if you can avoid it. The risks here are huge: if you lose control of one, And google decide it's tainted, you may lose control of the other.

Outlook.com may be a sensible alternate. Low barrier to entry. I am in pobox.com and have used fastmail (which owns pobox now) which has some upsides: pobox is a redirection service not really a mailbox but it provides carrier-independence.

Don't home it in an ISP email box you can lose, if you stop using that ISP.

Get a Proton Mail on their free tier.

Try hotmail.

I’ve been a satisfied customer and it works reliably, for most of my purposes..

Don’t use it as your main mailbox, though. There are glitches sometimes. It works well for a recovery e-Mail account imho.

For Gmail's recovery mail I use an address on my own domain, but for the domain's registrar I use an email at Yahoo! (mail.yahoo.com).

Yahoo! hasn't been without problems. I was annoyed when they deleted the inbox of users who hadn't logged in for a long time, with no way to recover the messages. That's quite bad if you're using it as your regular mail or as a long term archive. But as a rarely used location to get transient mails, the mail interface is ok and my account is still there after more than 20 years. Like Gmail, it's possible to fetch the contents over POP3/IMAP.