Method: identify the exact context your input lands in, then craft the payload that breaks OUT of that context. Prove execution by making your injected JavaScript call /m03/api/solve/filter (e.g. with fetch()). No external collector needed. Server-rendered sinks also auto-mark once a value that would run is reflected/stored here.
The filter strips one specific tag and nothing else. HTML has a lot more than one way to run script - reach for a tag (and event) the blacklist never heard of.