First rule of Home Automation: The model number matters
A zigbee 3.0 outlet may not help you at this time. The V3 version of the hub has some support for the 3.0, but the other hubs do not. Smartthings uses the “ZHA” ( zigbee home automation) profile instead. Although the Zigbee 3.0 devices generally will also work with ZHA, not all do. So it just depends on the specific brand and model that you purchased. (first rule of home automation: the model number matters.)
In addition, the Xioami devices are only intended to be used with their own gateway, and are not 100% compliant with the specification.
Setting aside the Xioami devices for a moment, Pretty much any mains powered device will act as a repeater. For zigbee with SmartThings, that usually means either plug-in pocket sockets or lightbulbs other than the Sengled bulbs. But light switches, in wall receptacles, and wall micros, and plug-in sensors should typically also repeat.
One of the best zigbee repeaters at this time is also one of the least expensive: IKEA Tradfri
The simplest and most reliable at this point (April 2019), which also happens to work well with the Xioami devices, are the IKEA Tradfri Plug-in module and their smart bulbs. (The handheld buttons and remotes have only partial integration with SmartThings and don’t work as repeaters anyway, since battery powered devices do not repeat.)
Those are available in most countries, and are quite inexpensive, so I would recommend starting with those. ( The product description will say that it needs its own gateway, but you can use the smartthings hub instead.)
https://m2.ikea.com/us/en/p/tradfri-wireless-control-outlet-30356169/
Rebuilding your network after you add a new repeater
As far as rebuilding a zigbee network with smartthings, that’s easy. That’s called a “heal.“ leave all of your other devices on power, but unplug the hub (including taking out any batteries) and leave the hub off of power for about 20 minutes. This will cause the other devices to go into “panic mode.“ Then when you put the hub back on power, all the individual devices will rebuild their neighbor tables. This can take a little while so you might not see improvements until the next day.
More Details
The following two FAQs should help you understand this better. Start with post 11 in the following thread, read that, then go back up to the top and read the whole thread. (The topic title is a clickable link)
And for more details specifically about repeaters, read the following: