ILLA cloud (https://github.com/illacloud/illa-builder)is another very good alternative to retool. We are 100% free for you to self-host your projects. We have a well-designed built-in component library for you to drag and place to build your tools quickly. ILLA is also trying to expand our collaboration with many open-source projects and databases.

Repo here: (https://github.com/illacloud/illa-builder)