A subdomain will be seen as a separate website by Google, so you might as well create a different website for every trainer and link to it from the main website to give them a DA boost.

If it's not working for you, then don't do it! Focus on the outreach methods that are working and delegate the rest.

As a fellow introvert, I understand where you're coming from and I hate cold calling. That's why I hired someone from the Philippines who does it for $8/hr.

If you don't have any budget at all, let other people do the work for you and give them a commission.

Sales representatives, affiliate marketers and referrals are all great ways to increase sales without sacrificing your mental energy/health.

I wish you the best!