Welcome to the forum. First of all, nice setup! (MQTT + Telegraf + Influx2 + Grafana)
What you want is certainly possible (I had / have the same issue with Flux, where I get crazy long legend names). I would suggest you not mess with the Flux value tag and just use an Override in Grafana. In my screenshot below, the original name was Memory (yours would be value {host="raspi", topic="sensors/bk/temp"}) and the new (override) name is Temperature Balcony
