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.

A lot of clients that I've worked with will purchase multiple domains for their single website. I always recommend doing a 301 Redirect to the primary ___domain name with all of the secondary domains.