I’ve worked with a couple communities that moved from Discord to Discourse, but both with a much older audience and likely a bad fit for Discord in the first place. Getting a teenage audience onto a web-based community sounds like quite a challenge.
Beyond the concerns you mentioned, do you have direct improvements for your members that you could offer on a forum, but not on Discord? You could also reach out more directly to your members to get a feel for that.
An inspiring example for a web-based gaming community is https://metafy.gg. They have a range of unique value propositions. But their platform is custom built and as one can see, with a ton of resources invested.