Well... here it is. It probably won't make sense at first to most people.
https://docs.google.com/spreadsheet...7-vOFwRSC35sbaonI7vvp58_HQ/edit#gid=500360676
Going down the sheet, the sheet gives a summary of loot by size and security level, then by security level for all settlement types, then the loot drop statistics for each specific settlement type.
To start off there's this generalization, REGARDLESS of SIZE:
low security settlements are good for specialized legacy firmware and modified consumer firmware.
medium sec give cracked industrial firmware, security firmware patch and MODIFIED EMBEDDED FIRMWARE; no specialized legacy firmware to be found here.
it's harder to generalize high security settlements, they are mostly like med. sec. with higher quantity drops, but if you are going after those you better know what you're doing.
One issue is that this data is from many many users, so some stats have reported drop rates of ~0.05 which is from a single reported case. I suspect these are from errors in selecting the settlement type. If you ignore these low-chance cases, what comes from where is more clear.