I'd consider a separate modem and router. That way you can get the best in both for your budget, yet when one fails or becomes underpowered/obsolete, you can swap it out, and still have half your investment still running. At least in the US, the modem-router-wifi units are built as cheaply as possible for sale to the ISPs, where there really isn't any consumer choice. If your TD-W8970v1 is working as a modem, then maybe unloading everything but the modem function will help.
I have a TD-W8980 myself and I use some experimental patches, which have been tested thoroughly and they improved the wifi a lot and also the overall cpu performance and ethernet speed. I use it with a external hdd for torrents and samba mainly because Internet is routed from HH5A. Btw you can also try these patches and hopefully they can improve the performance of your router in general.
At first it was thought that your flash may be corrupted so for that I would choose the overlay option with slightly changing the overlay part. At boot router will need to mount the USB and then, unless restarted, everything runs from the USB. I just modified the copy command to include rom and overlay.
tar -C /rom -cvf - . | tar -C /mnt/upper -xf -
tar -C /overlay -cvf - . | tar -C /mnt -xf -
You just need to execute the above commands in order and then everything should be in USB and your router should work fine.
I just realized that I haven't tried LEDE in a while and never started doing the whole git bisect thing... I rebuilt an image with the same "verbose-as-fuck" i used for master builds and kept the weird network and USB-overlay configuration. The router is still running and I haven't got a crash in 15 hours... And I begin to see things in the crashlogs!
I see this (all lines were starting with <date> kern.warn kernel: [<timestamp>]):
Yes it is a lede image, but its the only thing that seems to not crash within 5hours xD If it was already fixed then they may backport it, or point me to its fix, I could try to backport it and try to tackle the next bug...
It doesnt look much mandatory to fix if router is still working for you. It could be just some variable over-flowing. If the build works then stay on it and dont think much about fixing stuff because you cant just backport the entire 18.06, that will be pointless. I'd suggest to just use it as is and maybe wait for 19.x builds.