FYI the .trx file is stored in a subdir from the dir that has the start.sh script.  startshdir/trx_archive/creationdatetime

I have found out it's dangerous to just use the .trx.

"The firmware file must be divided into several parts before flashing. It is dangerous to do it yourself."
If no one can confirm it's safe to share the settings between the mi and the mi-3 (backup and restore) I suppose I'll have to do them manually. 

The following script (ran from the router) will create a file that contains the full config for the router in text file format with nvram set commands that can be edited so it is suitable to flash to a new router.  The normal binary backup is very dangerous because it will try to assign hardware addresses and other things so you'd want to delete anything except the section you wanted such as the ip ranges, static ip addresses etc....

#!/bin/bash

nvram showall > nvram_settings.txt
rm  nvram_settings.tmp
cat nvram_settings.txt | while read i;
do
o=${i%}
echo "nvram set " ${o} >> nvram_settings.tmp
done

sort nvram_settings.tmp > my_settings.txt

For which router?  The MI-3 required me to jump through some hoops to get ssh access.  You have to install the current developer version (which does not have ssh) then you have to pair your router to a xiaomi account via the android app.  Finally you have to download a ssh patchfile and save it to a flashdrive then hold down reset when plugging in power to router and continue to hold down until the router blinks the yellow light multiple times (not just once...it does that early and if you continue holding then it finally blinks a couple of times).

I don't think providing you MY downloaded patchfile would work since it's tied to my router.
Instruction originally from:

https://wiki.openwrt.org/toh/xiaomi/mini

Not recommended and may not work....again it flashes multiple partitions and you have no way of knowing if one of the other partitions needs an update for the new version you are flashing which the trx would not provide...I just let prometheus handle the flashing.  Currently on  3.4.3.9-099_6f8655c

More than likely you picked the "stable version" as opposed to the "test" version.

U only need one backup and u can move the backup to other location.

The backup should be your on PC and not your router if you are using the scripts.

It depends. I'm on time 100mbps.
Currently using WS880 as main router and Mi-3 as repeater.

Depending on test server, speedtest via 5.8ghz AC still can give me DL: 9X UL:9X Mbps.

Then occasionally I get DL:9X UL:6X Mbps but my another WS880 will still give me full speed. I'm sure it is not repeater here as I use Mi-3 as main before.

My suggestion is to continue use Xiaomi, unless you have a lot of device connecting to it. AC68U is good options when mainly devices connected and also support AC wi-fi.

Just my 2-cents.

here, https://mega.nz/#!dVAnBZ4A!V5A26H1a...bDYz673suwhnWnU

#!/bin/sh

### Custom user script
### Called after internal VPN client connected/disconnected to remote VPN server
### $1        - action (up/down)
### $IFNAME  - tunnel interface name (e.g. ppp5 or tun0)
### $IPLOCAL  - tunnel local IP address
### $IPREMOTE - tunnel remote IP address
### $DNS1    - peer DNS1
### $DNS2    - peer DNS2

# private LAN subnet behind a remote server (example)
peer_lan="192.168.2.0"
peer_msk="255.255.255.0"

### example: add static route to private LAN subnet behind a remote server

func_ipup()
{
#  route add -net $peer_lan netmask $peer_msk gw $IPREMOTE dev $IFNAME
    route add -net $peer_lan netmask $peer_msk gw $IPREMOTE dev $IFNAME
  return 0
}

func_ipdown()
{
#  route del -net $peer_lan netmask $peer_msk gw $IPREMOTE dev $IFNAME
    route del -net $peer_lan netmask $peer_msk gw $IPREMOTE dev $IFNAME
  return 0
}

logger -t vpnc-script "$IFNAME $1"

case "$1" in
up)
  func_ipup
  ;;
down)
  func_ipdown
  ;;
esac