Appointment reminder emails via SendGrid get rejected by Outlook/Hotmail addresses only

Mostly not your fault. On the free plan you share IP addresses with many other senders, and Microsoft has blocked that IP range (S3150 = IP on their block list). Your template doesn't matter for this one.

Options:

• Ask SendGrid support, they sometimes move you off a blocked pool.
• Switch provider for transactional mail (Postmark, Resend, SES). Postmark in particular is strict about transactional-only, so its shared IPs have a good reputation with Microsoft.
• Dedicated IP only makes sense at real volume, not for a practice.

Keep your domain authentication and DMARC in place wherever you go.

You can also request delisting from Microsoft directly (their sender support form), but on a shared IP someone else will get it listed again. For patient reminders I'd move to a provider with a better shared pool, as lena_ops says.

After switching, sign up for Microsoft SNDS and JMRP if you ever get a dedicated IP, and look at the DMARC aggregate reports for your domain in any case. They show you who is sending as you and whether it passes.