I'm the developer of GhostGo
This site has actually been running for many years. Recently I added a lot of features, and now there is not only a large number of problems, but also the ability to make problem, generate exercises, bookmark specific problem, an online sgf editor, and even include a simple forum and some related features for Go teacher. All resources and features and future features are free.
Site: https://ghost-go.com
Problems: https://ghost-go.com/problems
Editor: https://ghost-go.com/editor
Hope this helps.