As I want to rely on 2.4/5 GHz auto-selection for most devices at home, I couldn't test 5 GHz separately using my tablet as no option exists to force-pick an AP from multiple APs with same SSID on iOS. After a Web search, I found the creative workaround: just ban its MAC address at the 2.4 GHz AP. The speed is still inconsistent, as the reception became even worse - with an RSSI of only -79 dBm. The 250 Mbps PHY rate is also not much better than 150 Mbps at 2.4 GHz. But... the baseline improved from 3 MiB/s to 5 MiB/s, with bursts to 8 MiB/s. In the end, 80 MHz is overall a net win in comparison to 20 MHz.
I just discovered that I made a silly mistake with the OpenWrt One: not setting a regulatory domain. I thought it would default to the manufacturer's regulatory domain (sometimes this loophole allows people to abuse the lax US power limit elsewhere). Not with this chipset, which properly implemented the World domain with the lowish TX power at 5 GHz. Many 5 GHz channels are also blocked by default. After setting a real regulatory domain, the 5 GHz RSSI is now comparable to 2.4 GHz. I also moved away from Channel 36 @ 160 MHz to Channel 149 @ 80 MHz (which nobody uses as they all want 160 MHz), now running at 200 Mbps down and 80 Mbps up with a tablet. Now the 5 GHz AP is finally a real upgrade.
