Since WP Ultimo is supported for WaaS networks, it would also be helpful to accommodate database scaling strategies like:
1. HyperDB or LudicrousDB for sharding multisite subsites into multiple databases and for reading from a replica. Since WordPress creates ten tables per subsite, this is a performance bottleneck for large networks and sharding becomes recommended, if not totally necessary.
2. External databases (e.g. Digital Ocean Managed Databases, which makes read-only replication very easy at a relatively low cost)
These things apparently can be done now but with caveats (backups error, SSO failure, etc.) and there may be other uncertainties. Accommodating, if not specifically supporting, a setup like this might mean publishing which features will not work (ie. run your own backups, cannot use SSO, no Redis Object Caching, etc.) and allowing them to fail gracefully without error or replace with a notice reminding to disable.
LudicrousDB & Hyper DB WordPress Drop-ins
Scaling WordPress Databases | Remote Servers, Sharding, and Replication
Scaling WordPress with MySQL Replicas and HyperDB
Advanced Database classes for WordPress
Moving Half a Million Database Tables
I was unable to find server management suitable for a large multisite network until I found GridPane. The main issue was with SSL provisioning for subsites. Server Pilot and Runcloud use a multi-domain certificate from Let's Encrypt which means a network hits the wall at only 100 sites. I was so happy to learn that GridPane issues a certificate for each aliased domain, so there is no wall. I'd *love* to see the wall torn down for large network databases, too.
Thank you for already doing so much for WaaS customers.
I should mention also that my sharding setup is on a single DO managed database cluster. I don't anticipate grouping database shards onto separate servers but that is a possibility.
And I want to share a link to a post I made in the FB group in case others in a similar boat as me share there experiences with scaling multisite there: www.facebook.com/groups/selfmanagedwordpress/permalink/3546653488761899/