There is no truly all-in-one Home Assistant device off the shelf, but the HA devs have two reference kits that are mostly ready-to-go: Home Assistant Yellow and Green. There’s not a ton of difference between them at a high level, other than that Yellow includes a Zigbee radio onboard while Green does not. You can easily add other radios via USB dongles for Zwave (and/or Zigbee in the case of the Green) as needed. Both the Green and Yellow are supported directly by the Home Assistant devs and have lots of documentation. Home Assistant itself can easily replace SmartThings in terms of functionality, and in fact greatly exceeds it. Most of the headache is just gonna be migrating your stuff from ST to HA and the associated learning curve, which there is unfortunately just not an easy way to do AFAIK other than manually porting everything from ST->HA.
If you want a standalone hub with its own ecosystem most similar to SmartThings that you can later also integrate with Home Assistant, maybe the Xiaomi hub is the best option. However, it’s Zigbee only and mostly only works with their devices. You’d be better off just moving everything to Home Assistant up front IMO.
The message you’re reading applies to the checkbox above for encryption, not the preferences url. The preferences key only needs to be set if you want to encrypt the configuration URL, it doesn’t affect what OP wants to do.
My memory is a bit fuzzy because I switched to Searxng after playing with Whoogle briefly, but I thought Whoogle stored preferences in a cookie or something similar; the preferences URL is for when you want to transfer the preferences for your current machine to another. So OP is misunderstanding what it’s for.
OP: if your preferences aren’t sticking, are you maybe blocking cookies entirely or something? I’m pretty sure you shouldn’t need to do anything with the preferences URL for your preferences to stick if everything is set up correctly, it’s only for transferring your preferences to another machine.