I agree with @renojim that the additional device is probably best route to take. I am not far behind in doing the same thing myself as my Insignia is getting slower and more sluggish with each new OS update.
I just Factory Reset mine a few days ago to try and see if any better performance could be achieved but same behavior exists. Likely just running up against memory limitations.
If you want to try once more, I would put the original (or maybe the second) board back into the TV, and Factory Reset again. (Do not use your new working board). Also, remove any peripherals like other devices and USB sticks plugged into the TV ports. This time hold the Reset button until it lands on the Language Screen. (about 30-40 seconds). Then proceed through the setup and select "No Connection". (do not let it connect to your internet connection). After it finishes setting up, perform a System Restart. (Settings/System/Power/System Restart). You should now have a dumb TV if everything reset itself.
I would then try a manual USB update from tvupdate.roku.com
https://support.roku.com/article/212361468 (How to use USB to update your unconnected Roku TV).
[The above did not work for me on my Insignia RokuTV as my TV complained that the update file was incompatible, so who knows if this route is still working or whether the update file for my TV was corrupt/outdated, but worth a shot].
If the above USB update does not work for you either, then I would try to connect to a mobile hotspot (if available to you) in order to complete the required update rather than your internet connection. (Basically, trying to eliminate possibility that the Roku server is having a problem with the way you are establishing a connection, ie. your network and internet connection to the Roku server , as a contributing culprit).
Both the USB method and the hotspot should give you alternate ways of updating the TV.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941