I see a few things, best solutions in this order: preventing the wet, getting it dry, deodorizing, washing.
1. Preventing excess sweat build up, puddling, etc.
-Cool, dry air - a dehumidifier or air conditioner may help depending on your situation.
-more airflow. Tilting your fan down a little from where it is in your picture... aim it approximately at your crotch, it will cool your legs significantly which will decrease the sweating of the upper body as well, and then the air will flow past your whole body. You may have to fiddle with the aim until you get it where you like it.
-I plug the fan into a power bar next to my bike so I can warm up and then turn on the gale force winds.
-I also have a second fan that blows more on my upper chest and face.
2. The ventisit will help a lot. Air flows better through it than anything else. Indoor I ride shirtless with the thinnest indoor jersey slipped over the seat so it only has one very thin layer between the ventisit and my back, the other side of the jersey is below the seat and may catch air from the fan.
3. Getting everything dry is more important than washing. That is, if everything is dried well you won’t have to wash as often or as hard.
4. I have a Odour Eliminator from Norwex that I spray on the seat and in my shoes then place my shoes in front of the secondary fan set on low which dries everything.
5. Every now and again, maybe once a month, or if things smell a bit funky I take the seat pad in the shower with me and rinse it thoroughly and wipe down the seat and very occasionally put the shoe soles through the wash in a delicate bag.
In addition to the head rest holes there are holes in the frame beneath the seat-back.
Thus endeth my two cents. Hope it helps.