here's what I would have done differently

m2d/converters/event-to-message.js

@@ -158,6 +158,31 @@ async function getMemberFromCacheOrHomeserver(roomID, mxid, api) {
 	})
 }
 
+/**
+ * Splits a display name into one chunk containing <=80 characters, and another chunk containing the rest of the characters. Splits on
+ * whitespace if possible.
+ * These chunks, respectively, go in the display name, and at the top of the message.
+ * If the second part isn't empty, it'll also contain boldening markdown and a line break at the end, so that regardless of its value it
+ * can be prepended to the message content as-is.
+ * @summary Splits too-long Matrix names into a display name chunk and a message content chunk.
+ * @param  {string} displayName - The Matrix side display name to chop up.
+ * @returns {[string, string]} [shortened display name, display name runoff]
+ */
+function splitDisplayName(displayName) {
+	/** @type {string[]} */
+	let displayNameChunks = chunk(displayName, 80)
+
+	if (displayNameChunks.length === 1) {
+		return [displayName, ""]
+	} else {
+		const displayNamePreRunoff = displayNameChunks[0]
+		// displayNameRunoff is a slice of the original rather than a concatenation of the rest of the chunks in order to preserve whatever whitespace it was broken on.
+		const displayNameRunoff = `**${displayName.slice(displayNamePreRunoff.length + 1)}**\n`
+
+		return [displayNamePreRunoff, displayNameRunoff]
+	}
+}
+
 /**
  * @param {Ty.Event.Outer_M_Room_Message | Ty.Event.Outer_M_Room_Message_File | Ty.Event.Outer_M_Sticker | Ty.Event.Outer_M_Room_Message_Encrypted_File} event
  * @param {import("discord-api-types/v10").APIGuild} guild
@@ -179,6 +204,13 @@ async function eventToMessage(event, guild, di) {
 	const member = await getMemberFromCacheOrHomeserver(event.room_id, event.sender, di?.api)
 	if (member.displayname) displayName = member.displayname
 	if (member.avatar_url) avatarURL = utils.getPublicUrlForMxc(member.avatar_url) || undefined
+	// If the display name is too long to be put into the webhook (80 characters is the maximum),
+	// put the excess characters into displayNameRunoff, later to be put at the top of the message
+	let [displayNameShortened, displayNameRunoff] = splitDisplayName(displayName)
+	// If the message type is m.emote, the full name is already included at the start of the message, so remove any runoff
+	if (event.type === "m.room.message" && event.content.msgtype === "m.emote") {
+		displayNameRunoff = ""
+	}
 
 	let content = event.content.body // ultimate fallback
 	const attachments = []
@@ -364,13 +396,13 @@ async function eventToMessage(event, guild, di) {
 		pendingFiles.push({name: filename, url})
 	}
 
-	content = replyLine + content
+	content = displayNameRunoff + replyLine + content
 
 	// Split into 2000 character chunks
 	const chunks = chunk(content, 2000)
 	messages = messages.concat(chunks.map(content => ({
 		content,
-		username: displayName,
+		username: displayNameShortened,
 		avatar_url: avatarURL
 	})))
 
@@ -379,7 +411,7 @@ async function eventToMessage(event, guild, di) {
 		// There needs to be a message to add attachments to.
 		if (!messages.length) messages.push({
 			content,
-			username: displayName,
+			username: displayNameShortened,
 			avatar_url: avatarURL
 		})
 		messages[0].attachments = attachments

m2d/converters/event-to-message.test.js

@@ -1436,8 +1436,8 @@ test("event2message: overly long usernames are shifted into the message content"
 			messagesToDelete: [],
 			messagesToEdit: [],
 			messagesToSend: [{
-				username: "I am BLACK I am WHITE I am SHORT I am LONG I am EVERYTHING YOU THINK IS IMPORTAN",
+				username: "I am BLACK I am WHITE I am SHORT I am LONG I am EVERYTHING YOU THINK IS",
-				content: "**T and I DON'T MATTER**: testing the member state cache",
+				content: "**IMPORTANT and I DON'T MATTER**\ntesting the member state cache",
 				avatar_url: undefined
 			}]
 		}
@@ -1468,7 +1468,7 @@ test("event2message: overly long usernames are not treated specially when the ms
 			messagesToDelete: [],
 			messagesToEdit: [],
 			messagesToSend: [{
-				username: "I am BLACK I am WHITE I am SHORT I am LONG I am EVERYTHING YOU THINK IS IMPORTAN",
+				username: "I am BLACK I am WHITE I am SHORT I am LONG I am EVERYTHING YOU THINK IS",
 				content: "\\* I am BLACK I am WHITE I am SHORT I am LONG I am EVERYTHING YOU THINK IS IMPORTANT and I DON'T MATTER looks at the start of the message",
 				avatar_url: undefined
 			}]