This is just my personal experience and not that of all the experts on the forum. But, I have a FGM!
1 The FGM will heat up more slowly than some ovens owned by other people on the forum. The walls are insulated very well and that could make it take longer. On the other hand, I think it retains heat into the next day and beyond very well.
2 The dome thermometer (that inserts into the oven) measures the temperature at the core. Very different reading than the temperature on the floor or ceiling with an IR gun. The surfaces will get hot faster and easier but will also cool down faster than what you see in the core. That is why you saw the big drop off. The core was not really hot yet - you thought the oven was ready but it wasn't.
3, That does not mean you "have to" wait until the core is up to temperature before cooking. But, it means you have to manage it more carefully. Catch the temperature on the way down before it goes too far down. Spread coals back on the floor before cooking a second pizza in order to "recharge" the floor. I do not do this normally but have done so from time to time.
4. I heat up the oven steadily with a significant fire and the coals spread around until it is about 50 degree F short of my target. I then sweep the coals to the side for the side fire and sweep out the excess coals. At this point, the core is a little low but the floors are very high. If I wait about 20 or 30 minutes with a small fire on the side, the oven is ready to go. The core has moved up a bit more and the floors have cooled down some. So, it is all pretty evened out.
5. I figure about 250 degrees F per hour of heating. So, for a neapolitan with Caputo flour, 800 degree +++ pie, I will heat for about 3 1/2 + hours and then stabilize for 1/2 hour. I can probably start faster, but this has become my habit. If I "only" want to cook at 700 degrees, with a malted flour like KABF, then I will heat it up for around 2 - 2 1/2 hours and then stabilize.
If you heat up and stabilized as i described, you can pretty much make pie after pie after pie at around the same temperature by maintaining a nice fire on the side (that licks across the entire dome).
Others experiences may not be the same, but this is how I have learned to manage the heat up with my FGM. I am still learning every time I use it.
Great start and good luck. You will get better and better.