Also a good option
I'm giving adblock-lean a try. Not part of distro or package management system, but I like their project page: https://github.com/lynxthecat/adblock-lean/tree/master
I've toasted the flash on my previous router with adblock and too much writing to the filesystem, hoping to avoid that in the future. The adblock-lean project advertises "volatile memory used for all processing - no flash wear"
Nope, adblock doesn't write to flash fs by default.
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maybe I misconfigured it. $200 router toasted in 2 years. I learned my lesson about writing to flash. Wonder if I should try to replace the flash...
As a contributor to adblock-lean, I can assure you that adblock-lean doesn't even have an option you could change in order to make it write blocklist data to flash 
The only things written to flash are the script, the config file, an option in /etc/config/dhcp, a line in crontab and if you choose to install the optional utilities (gawk, GNU sed, GNU sort) then these are installed to flash as well by opkg or apk. Once everything is set up, no more flash writes (except if/when you update adblock-lean).
That said, I'm pretty sure that the other 2 native adblocking solutions for OpenWrt (adblock and adblock-fast) won't write to flash as well, unless severely misconfigured (I don't know if they have config options which would allow this).
The original flash has calibration data without which the router won't work. If you are able to read this data then most likely it is possible to replace the chip and make a usable image for the new one. If not then not. To try, you would need a flash programmer and some soldering equipment.