Look at Craft CMS multi site for a better implementation. It’s also multi-lingual, e.g. you can maintain Spanish and French translations for your global elements and then designate language for each of your sites and it’ll translate any shared content.

However, for 300 sites, anything but a custom web app will get out of hand because that’s too many to manage in a UI that’s primarily meant for a handful of sites.

I don't think there is any other option. If those are very simple websites, there are some wp plugins that allow you to map them to domains, i.e you can use a single instance of WordPress and map the content to different domains. One plugin for this use case is WP Landing Kit.

What’s shared between these 300 sites?

Drupal has an excellent multi site solution with the domain access module with which you can share content between multiple domains/sites.

I'll never understand why Drupal never became as popular as WP.

Yeah, it's weird. All the modules (plugins) are free and open source as well so it looks like a no brainer. It probably has something to do with the steep learning curve which can be intimidating.

I’ve seen drupal routinely implemented for larger clients, along with ezpublish and typo 3 at a previous job (i designed templates for these). The fact those were large projects that required multiple devs to implement led me to believe those were not a good fit for smaller projects/clients/personal sites.

I'm using it for a 10.000+ multisite installation (quite simple personal blogs), and even though it has it quirks it works quite well. Almost nothing is best in class and a slight pain to get it working, but the sheer size of the ecosystem is still it's biggest plus. In general every problem or feature you can imagine already exists or is easy to bolt on.

We've got a hundred tenant multisite running on WP and have been looking at other options over the last couple of weeks. I am most interested by PayloadCMS https://payloadcms.com/blog/how-to-build-a-multi-tenant-app-... . However our CMS needs, for this product, are very simple, so Payload suits.

We've also been looking at a multitenancy option for WordPress, in particular using Gridpave https://gridpane.com/kb/using-gridpane-multitenancy/ but you need the $5000 annual dev pro plan (annual for unlimited sites). We use Gridpane already on their lower cost plan and have found it excellent value for money.

We will probably go with Payload. WP is getting messier by the day. With the AI coding tools getting better and better it feels like having as much flexibility as possible will have a huge benefit down the line.