Dear Chris,
Here are my two cents in a sentence: "One wide and stable website that caters to multiple niches"
I am assuming that you wanted to book individual domain names and then redirect/forward them to the corresponding vertical sub-directories on the main domain.
For me, it would be a single domain any day and corresponding sub-folders/sub-directories for each of the verticals. IMHO, there is no need for you to book individual domain names that point/forwarded to the respective locations on the main domain.Even the URL like, www.abc.com/vertical-name serves the purpose and there is no need to search for the exact sounding root domain name like www.vertical-name.com and then point it to www.abc.com/vertical-name.
Getting exact match keyword domain names can be very difficult these days but even if you can manage to get your hands on one, due to the 301 redirection in place from it to the corresponding sub-directory on the main domain, the vanity URL won't make it to the search engines' indices and won't appear in organic search.
With single domain approach, all your SEO efforts can be consolidated and highly beneficial for the main domain from an SEO standpoint. Going forward it can become an authority. Single domains are far easy to maintain and operate from both promotional as well as technical standpoints. Each sub-directory's homepage can serve as the homepage of the vertical/niche. Each of these sub-directories and their internal pages can then have some good quality content optimized to rank for different keywords/phrases with healthy search volumes.
High quality content that covers the topic in a comprehensive way can have dramatic results in organic search ranking. Good quality content coupled with good promotional strategies with an aim to get domain mentions, citations, one-way backlinks is the key to good organic search ranking. These will result in healthy domain and page authority numbers.
All the very best to you my friend.
Best regards,
Devanur Rafi