In addition to what has already been mentioned, the GE Zigbee switches should be migrated first, to establish a strong Zigbee mesh. Then you can migrate the Sengled Zigbee bulbs. The Phillips Hue bulbs can stay directly paired to the Hue bridge, and their Alexa integration can remain unchanged. You can simply add the Hue Bridge to Hubitat using the built-in Hubitat - Hue integration.
As for the Harmony Hub, you can leave it directly integrated with Alexa. To integrate with Hubitat, you can use my Harmony Hub Integration.
You can then add the Hubitat Alexa Skill within the Alexa mobile phone app and expose the Switches and Sengled bulbs. Be sure to not create duplicates in Alexa for devices that are already connected to Alexa (I.e. the Hue bulbs and Harmony Activities).
