Managing multiple sitemaps for subdomains in Google Search Console

Hi All - I'm working on a site that has a handful of subdomains, but Google Search Console only shows the ability to add one sitemap (which is different than its help articles which say I can submit multiple.

The sitemap we currently have is just the root domain (although in GSC it shows verified for the whole domain property with subdomains), but the site is missing a sitemap for all the subdomains. What's the best way to handle this? Some ideas based on my research:

1) Use a sitemap index and list each sitemap separately

2) Combine them

3) Try to verify each subdomain in google search console (even though the root domain is a "domain property" in GSC, and will manage the subdomains)

4) figure out a way to add multiple sitemaps, which doesn't seem like an option in GSC right now.