Your signature construction for comparsion is incomplete
See this example for a correct one https://github.com/BarryCarlyon/twitch_misc/blob/main/eventsub/handlers/nodejs/receive.js#L37-L66
You need to .update with a string concat of the headers twitch-eventsub-message-id then twitch-eventsub-message-timestamp and then the raw buffer.
Your verification looks more like “old” Webhooks than EventSub