Trigger on POPULATION average? LTE router ping monitoring

[jtnfoley]

Zabbix 6.x, default ping template.
I have a population of cellular routers at remote sites that occasionally will all slow down (or enough to generate end user complaints from multiple sites...)
I built a sample population of sixteen sites, and have a dashboard graph that shows the average ping time across all sixteen. I'd like to create a trigger that will alert on a similar average, with either an absolute alert threshold or trendline slope threshold.
Has anyone built similar aggregated triggers?

[jtnfoley]

Solved.
I believe most of my confusion was (in part) that the tutorials and examples all seem to be from the Zabbix 4 era.
In 6, group aggregation is possible (optionally with a fake host with a default Agent type, and) using a calculated item that uses aggregate functions.
The Item type must be "Calculated" (not the z4 era "Zabbix Aggregate" type) with an arbitrary key name, and the formula to aggregate a 5minute average over the "Cellular Routers" group:
avg(avg_foreach(/*/icmppingsec?[group="Cellular Routers"],5m))

This is now giving me values I can trigger on.