Can't get temps to stick

I think you have the bottom too open. It also sounds like you are chasing temps. That is not a fun game. I think your probes are in the proper place. When you are doing low and slow you have to be very careful not to let the temp get too high. It takes a long time to bring it back down. Then you will be going up and down. (chasing temps)

The daisy half open meaning the petals or the whole top half open?  Either way, your settings seem really open for low and slow. What size egg?

250 on my large is, bottom vent less than 1/2" and daisy wheel, less than 1/8"

As @Dondc says, gotta be air-flow if you have sufficient fuel and a source of ignition.  The BGE uses air-flow to manage the fire volume and thus cook temps.  If all your indicators are to be trusted then you definitely are starving the fire.  Sounds like a good clean-out is in order, of course that is problematic  with some sort of fire burning right now.  
You can open the lower vent and remove any accumulated ash into a fire retardant container.  You can also use a coat hanger with a 90* bend , insert thru the lower vent and attempt to push it up thru your fire grate holes to clean them out.  FWIW-

What brand of lump?

You eventually learn the vent settings and learn to trust them. Patience is a must with egg cooking. Temp swings will always be a thing as this is a live fire. Once you get it down tho, it is a thing of beauty as it holds for hours. I’m still learning to trust the settings and let the egg do its thing. The more you mess with the vent settings, the more chasing you do. You’ll get there. 

One other thing to note is that the Maverick grate probe is going to read 20 - 30 degrees different than the dome.  I don’t use the grate probe very often anymore as just about all BGE recipes refer to the dome temp.  

@Hoops10, I finally learned to only use the dome thermometer to tell if my fire went out (and since getting a KAB, that never happens anymore either).  I learned where the vents should be for 250 (brisket, I could never maintain a 220), 350; and 425, 600 and 900 for pizzas, depending on style).  I never nailed those temps, but if you try to adjust you'll just get frustrated.  @Grandpas Grub had a good guide for settings on his Large that I used, but he hasn't been here in years and the pics were on Photobucket, so they're lost.  
 
You can get a blower setup or a Smobot, but those are expensive solutions (just got a Smobot myself).  If your temp is consistent, but 50 degrees off your target, that's really not going to make much difference in the end (my opinion).  FWIW.