Is it better to have an hreflang go to the home page in a different language if there's no corresponding page

[RoxBrock]

If I have some pages in English, but not in Spanish on my website:

Should my hreflang go to the home page on the Spanish site?

Or should I not have an "es-MX" hreflang for that page?

Ideally I would have all the pages translated, but this has not all been done yet.

[AxialDev]

If an English page doesn't have an equivalent Spanish page, it's best not to include hreflangs.

Hreflangs need to be bi-directional, meaning if site.com/en/a.html hreflangs to site.com/es/a.html, then site.com/es/a.html also needs to hreflang to site.com/en/a.html. This can't be done if you refer the homepage.

I'd make a note to add them later when pages are translated.