What you have mentioned is correct - to the best of my knowledge most suitable would be SNMP, but then if you have decided to go with WMI it shouldn't affect Solarwinds unless and until you have frequent health check up placed on your servers(Solarwinds Servers Health check + Solarwinds DB maintenance). WMI would definitely increase the load not just on Solarwinds but on the node side as well(the node that is managed by Solarwinds).
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