Just some suggestions from my pumpkin ale last fall. I’m not a huge fan of pumpkin beer, but others that are gave me pretty good reviews of mine. I thought the spice was a little heavy, but again it’s not exactly what I like in a beer, but others loved it.
For the pumpkin:
I used 4 of the big cans of organic pumpkin (15oz I think). 3 went in the mash. 1 went in with 10min left in the boil. The color was perfect but the pumpkin flavor was very light. Could have used anther few lbs. So I think going with 6lbs like you suggest is a good idea. I also like the idea of roasting it ahead of time. I did not and learned later this can really help bring out the pumpkin flavor.
I added 1lb of light brown sugar at the beginning of the boil. It will ferment out so you’ll get little to no flavor from it, but you will get some color.
I bought whole dried spices and ground my own mixture using a coffee grinder. I used allspice, clove, cinnamon, nutmeg & ginger. I added 3 tbsp with 10min left in the boil. For me, the spices were too strong. Others really enjoyed it. I would recommend using 1 to 2 tbsp in the boil and then trying the beer after fermentation. You can always add more later. If you like a lot of spice use 2-3 tbsp and if you want it toned down, I’d go with 1.
Hope I could help you out!