Another Linksys E8450 (UBI): attempted an attended sysupgrade from 22.03.5, replacing the old variant of wpad with wpad-mbedtls but otherwise preserving the list of packages. Result: no WiFi. Upon inspection, the reason why WiFi failed was the missing WiFi firmware.
The old version had the kmod-mt7615-firmware package, which contained the ROM patches for both mt7615 and mt7622, while in the new version, the mt7622 firmware has been moved to a separate kmod-mt7622-firmware package. Similarly, the mt7915 firmware is now in the kmod-mt7915-firmware package (was in kmod-mt7915e directly).
After installation of the missing firmware packages, the router works - including NFSv4, SAMBA4 (the real one, not the in-kernel version), PPPoE, SQM, USB LTE modem, modem goodies from @IceG, DNS over TLS via Stubby, DAWN, mwan3, StrongSwan for the Ivacy VPN, and a WireGuard site-to-site tunnel.