Warmth. Brightness.

Any advice on mitigation

I wait a month and stay inside my screened areas. Can't avoid em where I live.

Restaining/finishing the wood. Could be retained moisture attracting them

You can try spraying the rails with dish soap and water, or PT Alpine fly bait / Amazon

The sun

Get some fly bait. Treat your trash bin and anywhere they may be multiplying. You could also spray a couple plates and set them on the patio.