Update to this. Looks like the latest Debian stable kernel just wasn’t new enough (6.12.74-2), even with the latest firmware:
fw_version 0x1103006c fw_build_timestamp 2026-03-06 09:10 fw_build_id QC_IMAGE_VERSION_STRING=WLAN.HMT.1.1.c7-00108-QCAHMTSWPL_V1.0_V2.0_SILICONZ_UPSTREAM-3
After backporting the latest 6.19.10 trixie-backports kernel the card is behaving on 6GHz with zero issues.