I currently use dedicated servers and set up the GSuite Email on the CPanels
By adding a TXT record to your DNS records and adding MX records

Can this be done with TrulyWP, Have you done it with TrulyWP?
Is the fact that each site will have its own unique TrulyWP subdomain, going to be an issue for this?
you also mention "We allow outbound emails, although not unlimited, from each of your sites by default" - I am guessing but want to check whether or no your limitations come into effect since it is going through GSuite as the Email Client

Most of my clients are law firms or in the legal industry, I want to be sure of the transition and limitations before I consider stacking and migrating

3/
You mention we can not download backups, my contracts bind me to having backups in case we need to switch servers on the fly.
- I currently use WPMUDEV Snapshot Pro, referring to question 1, will running this plugin be an issue?

Thank you in advance for your reply