Following BIOS update to v1.16.0, I revisited the patched DSDT since it was (finally) also causing a reset to me. I dumped all tables from BIOS and came to realise I had applied the brightness keys patch to a Clover pre-patched DSDT!
I therefore re-applied the patch to the raw DSDT. My 7490 now boots Catalina without reset and brightness keys are fully operational. I've also updated the patch code above to return those Clover pre-patched lines to full vanilla code (XOSI returned to _OSI and XSID returned to OSID).
Patched_DSDT_Lat7490_BIOS-1.16.0.aml.zip
I invite Latitude 7490 users to test this revised patched DSDT and feedback to me by PM so that I can update the bootpacks accordingly. In case of issues (system reset), select BIOS.aml DSDT table in Options->ACPI patching->Dsdt name at Clover main boot menu to load the raw DSDT at macOS startup.