Freetrade are doing a lot to build a community and knowledge base. But a drip email marketing campaign might be a good way to test point 1 from your suggestions list. It could also be a good way to group members on the same point of their journey, and see what/how people drop off etc. There’s a lot you can do with email these days.
NinetyNine have gone some way to build a learning section and it seems well structured.
In terms of other ideas, I like Kenny’s
(edit) - Robinhood has Invite Your Friends, Get Free Stocks - I don’t know how well it’s working for them but it’s interesting nonetheless, suppose their forecasts show their LTV can support this.