Well done 
. That's exactly how I would do it. But why weren't you able to select to boot from tftp? Maybe you didn't enable the waiting time for the boot selection?
That's done by using nvram set bootdelay=5
Btw.: There's also an official documentation how to recover back to stock without the MIWIFIRepairTool: https://openwrt.org/toh/xiaomi/xiaomi_redmi_router_ac2100#stock_recovery
However that's much more complicated as you need a dnsmasq and a tftp server. So using the original recovery tool is much less work.
For everyone who needs to follow the instructions for the easier miwifi repair tool you can find pretty nice and simple instructions with screenshots here: Xiaomi Mi Router 4A Gigabit Edition (R4AG/R4A Gigabit) -- fully supported and flashable with OpenWRTInvasion (just don't use the image file which is mentioned in there as that is for another router model - everything else is the same).
Which package install broke your image?