The 32 device limit is the total number of battery-powered zigbee devices (plus Sengled Element lightbulbs). Not just one device class. But every mains powered zigbee device ( except again Sengled element lightbulbs which are in the other group because they don’t repeat) can take some of the burden off of the hub.
The Lowe’s Iris smartplug 3210-L Is popular because of the unusual fact that it includes both A zigbee repeater and a zwave repeater.
https://www.lowes.com/pd/Iris-120-Volt-White-Smart-Plug/999925330
https://www.amazon.com/Iris-Smart-Zigbee-Works-Alexa/dp/B01GCKAED4
Not the other Lowe’s smart plug models. Just that one.
Otherwise, any Zigbee pocket socket that works with SmartThings should act as a repeater, so those are popular. 
However, that’s not what’s causing the problem you describe as it would only affect non repeating devices of the zigbee protocol. And it wouldn’t happen at the same time or thereabouts each day.
The problem you described is one of the hub becoming unavailable To all locally connected devices. Not of a few individual devices not being able to get through to it.
Also
Since I already have old ones in place just added more of the same, I did not consider this as a pattern change
Having four of something instead of three is definitely a network change. Adding a new device, any new device, is a network change. Moving a smart plug from one room to another is a network change.
Take a before and after picture. If they don’t match exactly, there’s a network change.