Ok, first, welcome. Second Hi everyone who hasnt seen me in a solid year or so. Life happens and you got chit to deal with. Dealt with, and I'm back.
Now, have you read the UDS thread to see the methods that have proved to be most successful, because judging from your post, you have not. I don't imply you have to read all multiple hundred pages, though I did, and when I have trouble with my UDS I know EXACTLY what it is.
3.5" exhaust is pretty big, most of us run 2".
Intake, 3, 3/4" inlets. All 3 open to fire it up, and two closed one adjusted as needed to dial it in. I generally run 1/2 open ball valve on one 3/4" intake and that keeps me in the neighborhood of 250 +/- 10 degrees.
Do you have legs on your charcoal basket? The basket needs to be elevated off the bottom of the drum for ash to fall through and air to get up. That being said, what material is that mesh made of? Zinc is toxic so make sure it's not that. There is a reason we use the materials we use, though they arent the cheapest available.
Have you looked into the burn methods that we use on the UDS? I personally use a minion burn, with great success. If you're not familiar, thats filling your charcoal basket to the capacity needed (you'll dial that in as time goes on) and lighting about 3/4 to 5/8 of a charcoal chimney red hot with charcoal then dumping on top of the basket and letting it burn down.
Next, PATIENCE. I try to stick to a 30 minute timeframe. Checking the temp 30 minutes after an adjustment. You will fight that intake open closed open closed all day/night if you cant be patient and let it level out.
And finally, ambient weather. Ambient temp, and wind are major factors when using a UDS. If it's pretty cold out, it'll knock down that temp pretty fast and you'll have to run with a more open intake and burn more charcoal faster. If it's windy, you have to take into account the direction of your exhaust and intake. You can create a vacuum in the uds basically if the wind is fast enough in the right diretion. It's wisest to have your intake facing away from the wind. I had my intake 90 degrees to the wind once and it completely extinguished my burn in a matter of a couple hours. Ruined a 30 dollar brisket. I hope the information helps you to narrow down where your problem is.
--My real name's Ty.