Fix broken link tracking due to HSTS on subdomain?
You can use a CDN such as Fastly or CloudFlare to perform SSL termination on your SendGrid click tracking links. They will terminate SSL (which will satisfy your HSTS config) and proxy the request to SendGrid for click tracking / redirection.
There is some additional information here:https://sendgrid.com/docs/Classroom/Build/Add_Content/content_delivery_networks.html
You essentially want to configure Fastly / CloudFlare to proxy the requests, reach out to SendGrid support to have them verify and enable "SSL click tracking" for your account. Once they confirm that everything is set up as expected, you can then update the CNAME on your subdomain to point to the CDN provider rather than to SendGrid directly.
You also have the option to set up Apache with mod_proxy to terminate SSL and proxy the requests directly to SendGrid.