I agree with SSdoxie. My DK fuel gauge is very slow to move. What you describe sounds normal; is the current behavior something new?
WRT glow, MY DK starts immediately after just a couple seconds of glow. Your double glow indicates a problem, IMO. If it were me, I'd sort that out before you get cooler weather. You may have some bad glow plugs. You may have a bad connection to the bus bar that connects all the plugs. You could have a problem with the ignition switch (that was my problem). You could have a problem with the glow relay/timer. Or it could be something wonky, like a bad wire or a corroded inline connector. If you can read a schematic and know how to use a voltmeter you should be able to isolate the problem fairly quickly. It helps to have a helper so one person can work the ignition switch while the other tests the voltage at various points. Start with the bus bar to see if the glow plugs are getting power with the switch in the glow and also in crank positions.