I've only worked a couple of solo HS matches, but they were freshman. The pace and skill of play will work to your advantage.
Hopefully by this point in the season they've got all the libero and sub kinks worked out. But without an R2, just keep an eye peeled by the benches between points.
Also, when working solo in any sport, I've found it helpful to be very strict on the little things, in this case like making subs wait every time for the SK's thumbs up, and end of set protocol.
It sends the message that you're watching everything and there will be no shortcuts or shenanigans today. But I'm sure you know that
