I'm surprised the author rejected using GitLab because of its Open Core model, but decided foe using Drone CI, which appears to be following a similar model.
My experience self hosting Gitlab was pretty terrible, it requires a lot of attention on your part, and internally the project feels like a bunch of fragile scripts.
There’s also little documentation for when things go wrong, with was almost every single update.
In the end I switched to Gitea as well and I’ve been happy with it for years at this point.
I've tried github actions, drone, woodpecker and gitlab. In my opinion gitlab has a much more flexible CI system.
Had two upgrades that needed minor manual intervention in 5 years. Uptime better than Github despite the #yolo approach.