I proposed Telegram because it's simple and has a huge network effect, tons of bitcoiners already use it and that's what you'd look for. You can buy a phone number with silent link, ignore their shitcoin and start using it today.

Nostr would be a nice alternative, but last time I checked group message discussions were still in development.

I also think matching txs might be too much of a hassle for a small 3 to 5% discount. Although if fees continue like this and you transact a lot on-chain it might add up.

Also I was just testing the waters, seeing if there would be any interest in doing this at all.