You get that error because Flent can't parse the output. In this case because the timestamp is missing the fractional second part.
Installing coreutils-date should fix this; I've bumped the flent-tools package version and added this and coreutils-sleep as dependencies.
Just a quick follow up. I did end up testing with flent after the above fixes but the results are not straightforward to me (I may be making it more complicated than it needs to be).
Regardless, shortly after some initial tests, my family requested "the better AP" be returned to regular service so I've stopped for now.
Ironically, it seems new issues related to the wifi on my r7500v2 have come up on master likely from multiple sources (ath10k-ct firmware, mac80211 upgrades, hostapd changes, etc.) not the least may be related to my own configuration tweaking (some adjustments to my VLAN config).
Regarding the testing with flent that I did manage to do, I'm uncertain about being able to measure a 5% cpu benefit "on overload" when the r7500v2 cpu is "maxed out" and I'm not somehow holding throughput constant. For example, I seem to reach a cpu limit (rather than say the switch or nics) if using "one" core (irqbalance turned off). However, if I turned on irqbalance, I can get an extra ~50 mbit/s through the r7500v2.
I hope to come back to this, but it could be a year or more given the way school for my children and covid 19 are looking ATM.