Google devalued exact match domains years ago, but it still makes a difference. 301'ing the subsequent domains won't help you rank unless they already have back

Google devalued exact match domains years ago, but it still makes a difference. 301'ing the subsequent domains won't help you rank unless they already have backlinks. If you build out each domain with unique content and generate backlinks to each domain, then link to your primary domain with your target keyword as the anchor text, you may be able to help the visibility of the primary domain while also taking up additional spots in the SERPs with the keyword rich domains. You should probably check out the monthly search volumes for the EMDs and see if they have any meaningful volume for the target terms. Anything under 100msv is probably not worth tons of effort.