You may want to hold off on Sengled outlets... I seem to recall that these are NOT Zigbee Repeaters.
You may want to invest in some SmartThings Zigbee outlets or IKEA Zigbee outlets instead to help build out a solid Zigbee mesh for your new Hubitat hub.
Also, the Sengled Zigbee bulbs work great with Hubitat because they are not Zigbee repeaters. Most other Zigbee bulbs, like Cree and GE LINK, cause problems with Zigbee HA mesh networks. These bulbs may be the cause of your current sensors dropping off your ST mesh network.