What is the impact of SEO on providing multiple custom domains for portions of our main site?

The provision of different domains is similar to the one you have with different subdomains. So, having business1.site.com and business2.site.com is identical to having business1site.com & business2site.com.

You will have to market each subdomain / domain separately, in order to obtain relevant links to each entity separately.

Each subdomain / domain essentially functions as a separate "enterprise" within society as a whole.

I would not recommend it, but if you insist on creating subdomains / domains, perform a 301 redirect, "permanently moved" to your main site, because, according to Google, this only affects 15% of ranks .