Hello, same problem solved this way :
I use var addon https://github.com/snarky-snark/home-assistant-variables.
with value_template which says
{%if is_state("sensor....", "unavailable") %} {{states("var.inklevel") }} else {{ states(sensor....") }}
Doing so the variable updates only when sensor is available. Works fine !
Of course the lovelace card shows the variable 