As far as what to do with it, I could picture house-wide ad/tracker blocking, sending traffic analytics to elastic, potentially some homeassistant/rtlsdr type stuff.
One thing to know is that the overall documentation situation is "mixed", and if you Google for how to do something with OpenWrt, you'll often find very old notes someone wrote about how to do something with the command line and editing files. (I know Linux pretty well, have used OpenWrt for years including some complex setups, and occasionally I have trouble finding documentation of simple things like, e.g., what partitions/mounts are currently volatile, for example, and distro-specifics of where you're supposed to edit packet filtering rules.) There's not enough correcting of that scattered documentation, such as to say that almost everyone just uses the Web admin interface now. When in doubt, just look at what the Web admin UI currently does, and try to use that.
The only reason I use the CLI is because I know how to and it’s quicker to backup the configuration (ssh root@<ip> uci export > backup) and restore it (uci restore < backup) but this can also be done through the GUI.
The GUI is a nice combination of ease of use and advanced features. You can configure the basics easily, but the advanced stuff is also there if you need it (and if not just keep the defaults which are sane).
See other people's comments on running Openwrt on the C7, seems to have issues with 5ghz WiFi?
I have openwrt on all my 3 Archer C7s.
I much prefer it to dd-wrt.
Curiously, neither one lets me flash 180305, which is listed as the current version on the Canadian support portal: https://www.tp-link.com/ca/support/download/archer-c7/v2/
This kind of thing is why I stuck with the stock FW so long, le sigh.
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Oh fun, so there's a note on this page about there being separate versions for US and rest-of-world: https://openwrt.org/toh/tp-link/archer-c5-c7-wdr7500
I grabbed the archer-c7-v2-squashfs-factory-us.bin file and we seem to be in business now!