OpenWRT/DD-WRT/Tomato are supported on a model case-by-case basis. It is not a given any random consumer router model will work with those opensource firmware replacement projects.
Also, has you have found out, Broadcom is (in)famous for their opensource support. What that means, it that OSes for supporting all their functionalities have to have installed/load binary firmware blobs provided by the manufacturer. Such position might be against the philosophy of some projects (OpenBSD, OpenWRT), so no versions supporting them.
Seeing the OpenWRT wiki about support for NetGear R7000 at https://openwrt.org/toh/netgear/r7000:
WiFi 2.4GHz partly, WiFi 5GHz
So OpenWRT only supports well Ethernet at the moment.
There is support for DD-WRT/Tomato/Advanced Tomato. There seems to be some stability issues in some versions, and no current support for the Tomato fork, so thread carefully. https://advancedtomato.com/downloads/router/r7000
Looking at DD-WRT page, there seems to be a DD-WRT firmware from 2017? https://forum.dd-wrt.com/phpBB2/viewtopic.php?p=1087382#1087382
Based on my limited research for this question I would probably go either for DD-WRT or Advanced Tomato.
I advise doing a more complete research and maybe some testing with both versions to ascertain which one is more attuned to your needs/preferences.
P.S I have a 802.11AC AP at home, that I shopped around expressly for working with OpenWRT, an Archer C2 TP-Link AC1750. However the harware is already a bit outdated by now.
My usual advice about WiFi devices is doing your research for intended software/ open-source support before buying them.