Great question—international SEO can definitely get tricky, but you're already on the right track by thinking it through early.
To answer your first question: yes, you should set up a separate domain property in Google Search Console for your .com site and submit a sitemap. This won't automatically cause duplicate content issues, as long as you've properly localized the content. Since you've already adjusted it to American English, Google should treat it as unique enough, especially if you tailor other elements like spelling, examples, pricing, or even imagery to suit a U.S. audience.
To help further, make sure you:
Set the correct target country in each Search Console property.
Use hreflang tags to signal the language and regional targeting of each version (e.g., en-GB for UK and en-US for the U.S.).
Avoid exact content duplication across the domains.
As for expansion into Europe, your concern is valid. Using subfolders like /fr is a solid approach for country targeting, but yes—this method requires everything to be under a single domain. Structuring URLs like www.domain.co.uk/fr isn't ideal because:
The .co.uk TLD signals UK targeting, which can conflict with your intent to reach French users.
It may cause confusion or impact rankings for other European regions.
If international growth is a big part of your plan, you might consider:
Best Option: Move everything to the .com domain
Use subfolders for each country/language (/uk, /us, /fr, /de, etc.)
Consolidates authority and backlinks on one domain
Easier to manage long-term from an SEO standpoint
Use hreflang tags to help Google serve the correct version
This is the SEO structure that international brands such as Zara use to promote products internationally.