10(ish) years ago, Netgear was my go-to recommendation. I don’t know of any netgear box produced in the last… 5 years… which supports OpenWRT easily (as in, download the right OpenWRT image and drag/drop it into the firmware update box on the router’s web interface). I believe there are a few Netgear boxes which you can get OpenWRT running on through more invasive means (including JTAG programming or similar).
With regard to dedicated router and separate modem: modern protocols (V-DSL, DOCSIS, and similar) are not well documented, and typically implemented in silicon (as they really require an ASIC to push the speeds they get over the physical lines they have). For the most part, attempts to get those ASICs working on OpenWRT or similar have been… temperamental at best. At the very least, they end up needing both a firmware blob and a blob kernel driver, which significantly degrades the trustworthiness of the system. If you are buying equipment up front (which you should do if you think you’ll want the service more than about 3 months), you can buy a dedicated just-a-modem, plus a TP-link or similar device, for about the same price as a slightly higher end all-in-one modem. Considering that the TP-link half goes with you across ISPs and is likely to continue to function for as long as the speeds it can manage are good enough for you, you’re money ahead this route the moment you switch services and get a new dedicated dumb modem.