My 3080 with 3900 had a similar problem. I solved it by using two extra fans positioned outside the case, tucked between the side panel of my table and a wall, just under the top desk. It takes the hottest air (coz its right below the desk plane) and blows it along the wall to the side. I have them running at, I think, 350 rpm (14 cm fans), so you cant hear them at all and it doesnt feel like any significant amount of air is moving, but the temperature under my desk dropped to comfortable, almost ambient levels.

The fans are, via extension cables included with extra fans I bought, plugged directly into motherboard, zhe cable pulled trough opening for second gpu connectors.