I’m asking because for this use-case Wordpress Multisite seems to be the only option, but I’ve also always despised its structure and inner-workings.
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.
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.