You don't say how the Aga is fed. Or if you have any way of gauging temperature of the ovens and hot plates. Mine is electric so I can reasonably rely on a constant temperature.
Have you had it serviced?
Get some way of checking temps n ovens
I have found bottom shelf tends to heat food faster due to heat from base of oven.
I would say WHAT you are cooking in is as important and what the food is and how it wants cooking.
We always cook roasts on a bed of veg with water in the bottom, cover for first half and uncover for second, take meat out and cover let rest for 15 20 mins but this is fairly standard. If its a large roast I try and use a heavier dish as I find this spreads the heat better and may also use the cold shelf under after the first 30 to 40 minutes.
Baking is more tricky. My Oven is set for around 180/190 which is a bit low for buns and a bit high for cakes. I do muffin size cakes in a 12 tin, but place near the top of the oven. You can check in Agas without effecting the cake too much which is a pro of using it so open door quick check and close isn't an issue. For actual cakes I tend to use smaller shallow cake tins and layer them up - this gives a smaller amount to cook so less risk of burnt edges. If you need a bigger cake I can turn my oven down but other things to think about are wrapping the tin with brown paper of some of that fancy greaseproof etc AND using the cold shelf after the first period (will vary depending on cake type. So anything that has to be in a long time thing about protecting it top and bottom as well as reducing the temp a bit if you are able to.
The other advice is anything that needs a higher temp e.g. yorkshires, cook them first while heat of oven is optimum - chuck a slab of meat in and it will effect the temperature.
I have found aluminum tins for cakes the best - I have a dinky pair of 6 inch ones that take up to a 3 egg mix that are perfect for the viccy sponge or choc cake. I also have a much bigger set and have successfully baked a 14 inch deep Madeira in one go on a reduce d oven temp and wrapping, plus using cold sheets up and down, but not for the full cook!!
So its def more of an art than a science.
For christmas - I generally cook a crown - although if you have too many could still be an issue - layer up with plenty of bacon but don't stuff the actual crown/bird - cook stuffing separately. Cook sitting on veg with plenty of liquid (can be used for gravy as its stock) and cover for at least 75% of cooking time.
Cook ham day before and again I cook in stock it seems to reduce saltiness and ensure its not too dry if its a big one.
Remember having the top up and veg on will take heat from the oven so maybe parboil roasties first thing and let go cold before poppin them in later on with the Turkey. With regard veg we do roasted parsnips (treat same as roasted pots) then rest of veg is steamed which at the longest takes 25 to 30 minutes on the top so it goes on 15 mins before turkey comes out. And cooks whilst turkey resting for a bit (covered to keep heat in) and carved.
Gravy made while bird resting and veg cooking as well, using juices from bird etc
Would you be able to have a bit of a trial run before christmas? Make notes to remind yourself how your oven works etc