I think the Hydros 21 takes half a second or so to warm up and take a sample, so it’s probably fine. It’s turbidity sensors, sondes, and anything Yosemitech that are really the slow-pokes.
If you’re not worried about the small amount of extra data consumed on your cellular plan by those few extra characters, you could also just include the extra variable and send it with the empty UUID just like you’re doing now. If you’re right at the edge of a price change on your cell plan, it might make a difference, but it’s not going to be huge. [~10ish characters * 1 byte/character * 288 posts/day (5min intervals) * 30 days = 0.0864MB/month per junk variable] A lot of the cellular data consumed is in overhead as the unit opens the connection to the server, does the TCP three-way-handshake, and repeats any packets that don’t get acknowledged by the server.