In some instances the operator has to define what to use.
In my outfit we use "wet" for all runways where some form of contaminant is present but below the limits of being considered contaminated.
Your special case mandates a "contaminated" calculation as I have to assume your "
5mm of loose wet snow" is present on more than 25% of the surface.
"Loose" is not a qualifyer used in RCAM or any reporting. Which put's you into the table for 5mm wet snow. For performance calculations you have to use the RCAM table from the AFM or operators, similar to this one (FAA - but it's pretty much the same for EASA)
https://www.faa.gov/about/initiative...Pilot-RCAM.pdf
It puts you into RCAM 3/3/3 which is at least "wet" or "contaminated". Usually performance tools are able to calculate with wet snow over 3 mm depth because it is considered a contaminant. Depending on the tool i will tell you a water equivalent or simply do a calculation based on actual TOM or LDM and the result is either green or red. (go or no-go)
As for a basic approach to contaminants: Anything which sticks to the surface of a runway is a contaminant, regardless of what it is. Therefore "dry performance" is already out the window. Would you be comfortable driving your car down a slope with 5 mm of wet snow and you assume "only" a wet road? Do you think you need more braking distance, or less than on a wet road? If all your tables don't tell you exactly what that contaminant is you have already pointed out that you may use 7mm wet snow from the AFM as you have a table.
When the RCAM first came up I found myself in a situation where snow over sanded ice was reported with braking action "MEDIUM" but at that time I had no legal table to do a calculation based on braking action. Therefore we calculated it with "wet-ice" which is braking action NIL and the calculation came out green with 80 m remaining runway. Legally good to go we landed and left the runway at the usual taxiway about half way down. 1'400 m autobrake MAX, full reverse for the first 10 seconds, then all off as we would have stopped well short of any useful exit. The point is - good airmenship.
Hope this helps answering your question.