Combining / merging 2 sites

I’m taking two sites — mysite.it (which is all in Italian) and mysite.com (which is currently all in English) — and combining them / merging them into one site — mysite.com (which will be bilingual Italian/English). At some point, I need to redirect all traffic from mysite.it to mysite.com.

I’ve been using Yoast for a while (thanks for a great product!) and have a good Google ranking for both individual sites. I am obviously keen not to lose the rankings on mysite.it while moving to the combined mysite.com. I also need to make sure I’m not competing with myself on the Italian content at any point.

I’ve taken the xml sitemap and I am building up a list of redirects from the old to the new. I’ve read Yoast’s page about canonical URLs. I’ve also read Google’s advice on moving sites in the Google search console. But I can’t find anything that specifically tells me how to sequence combining sites like this.

Do I:

1. Keep the .it site running for a while, with 301 redirects to the .com site? (I’ve used the Redirection plugin in the past, and would use it for this.) Or

2. Keep the .it site running for a while, with canonical URLs set in the Advanced settings of the Yoast SEO meta box? Or

3. Redirect the whole .it site from day one to the .com site at the domain level and hope for the best (in multiple choice questions, there’s always a stupid answer, right?).

I’d be nervous about (3) because some of the site’s structure will also be changing, with some pages also being split and merged, so I’d like some way of telling Google etc where the new info is. I mention this option because it would certainly make sure that I wasn’t competing with myself on the Italian content at any point.

(1) is my favorite option, although in the past I’ve seen a Google article saying that if you’re moving sites, don’t do it in stages, but get it done in one go (though the move scenario doesn’t match my case, as I’m combining sites, not moving them).

For (2), Yoast’s article on canonicals says that “If you are unsure whether to do a 301 redirect or set a canonical, what should you do? The answer is simple: you should always do a redirect unless there are technical reasons not to.” But again, this isn’t specifically addressing the issue of merging sites. Additionally, this approach would not allow me to redirect attachment pages properly (and as the site’s images crop up a fair amount of searches, they probably help push up the rankings too).

So to the question: from a Yoast/SEO point of view, is approach (1) better than (2) in order to maintain rankings? Also: Am I missing a better, completely different, approach?

Any advice very welcome. There seems to be a dearth of info on this.