Hi Bert - A slight diversion from your question of managing this via the exec binding…
I also had zelio’s managed via the exact same process , using a adaption of the same c program that @Jeroen_Ost wrote, which served me well for many many years.
When I moved to OH3, I started experiencing numerous issues with the exec binding seeming to choreograph all the item updates to overlap in the same millisecond as the others, causing intermittent errors/state inconsistencies. See OH3 Exec Binding with Serial Port - Port Locking due to overlapping item reads for details. I only had around 8 items…
I have just posted up a Zelio->MQTT service example (see: Zelio Integration to OpenHAB via MQTT but this is probably not much use to you yet, as I have not yet created the MQTT->Zelio (Write) function…
Just be aware of this locking issue with multiple items reading from the same device… I suspect you could run into this, once you solve your original question, especially with a one second update interval.